There are some errors that can ruin the fun for everyone because they result in certain defeat much earlier than it should or because they doom you later in the war. I will list the most common errors that can really ruin scenarios.
General error 1: Not reading the playbook section for your game, and all necessary pages in the rulebook.
If you are playing a training scenario(Poland, Scandinavia, France 1940, Main Event -1940) then the rules you need to know for that training scenario are listed in the trainings method section of the playbook. Not having even read the rules is not a great idea for a game with the maximum complexity score.
General error 2: Open Frontline(except as a reaction to an open frontline in your USSR home territory)
Leave spots open in your frontline in enemy territory? The enemy will say "thanks" and block your supply, isolate your units and (in the USSR) establish a second line behind you whilst regaining national will. Especially if you are in a disadvantage(e.g. Russian Winter)... try not to leave holes in your frontline. It can be messy really fast. I had multiple USSR 1941-1944 scenarios ending with an allied win in early 1942.
General error 3: Not attacking the air units as axis with air strikes before starting the attack.
Terrible. +2 for you and +2 for enemy isnt good for you(at least not in that situation) because high results are not as favourable for the attacker than low results.
Main game error 1:Bad event timing with Axis.
This error will ruin your game. If you trigger East Invaded or West Invaded at a specific time expect the last policy to fall 13 months later- or whenever you end it. Ending the last policy in July is good. August or September is a MASSIVE advantage for the Western faction later. That's because if you end the policy in July, the game ends four years later. In August, it ends four years and 11 months later. But the USA counters will still come a fixed time after the end of the last policy. Also important: With East Invaded you REALLY dont want to trigger it before necessary. It has the same implication as above, albeit less with units but more with surprise attack markers, factories, and the Partisans marker. Going into Lithuania, Estonia and Latvia(if possible) to push your frontline... no, just no.
Main game error 2: Unnecessary declarations of war
Do you need to declare war to achieve a goal? Go for it. Is it not the right time, you are busy elsewhere, or the benefits are not worth the extra strategic power for Soviet later? Don't. Similarly, as allies, don't declare war against something too early! Especially not Italy(it has an air unit that could help them with the invasion of the UK, Mediterranean or USSR)!
Main game error 3: Bad diplomatic decisions:
-Empty the pot as Allies
The axis players conquers a country, puts a pro-axis marker in a country as well in the pot, then uses that marker to get a guranteed activation of that country...
-Activate a weak, unnecessary country as Axis.
(Latvia, Estonia, Netherlands, Belgium, Denmark, - notably not Iraq in some situations because of the Sea Zone strategic combat bonus, and in very specific situations maybe Syria...)
-Activate a country as Allies except:
--Italy
--Soviet Poland(Wide Open, Appeasement has ended)
--Turkey(in this order, although if you can only activate this one, probably, go for it)
--a useful country if the diplomacy phase is expected to end very soon, especially if it is likely the last
--a useful country during a late Moscow Treaty
Allied Italy is extremely good for allies. It permanently denies Italy for Axis and disrupts all Axis strategies because they have to urgently secure southern Germany from an italian invasion... and because their plans for the Mediterranean are probably dead, freeing up allied units for other spots - like Mainland UK.
Turkey: two reasons. First, Mediterranean. If Axis Italy and then Axis Turkey activate the Mediterranean is probably lost.
Second, USSR doesn't want Axis Turkey either. Yes, it gives them two conditional units but it also gives Axis three full minor country units ready to attack at the southern flank! And they do not cost German production.
--as Soviet, ignore the possibility of an Axis Finland without a ceded area.
That's terrible for later, possibly. Instead of being able to quickly dispatch Finland via that area you might even end up with a conquered Leningrad... which gives Axis a direct USS line into the north and cleans up a potential point of resistance! However, if West Invaded has already been triggered and Norway, Sweden and Finland are neutral countries without an axis marker, you should not try to get Finland.
General error 1: Not reading the playbook section for your game, and all necessary pages in the rulebook.
If you are playing a training scenario(Poland, Scandinavia, France 1940, Main Event -1940) then the rules you need to know for that training scenario are listed in the trainings method section of the playbook. Not having even read the rules is not a great idea for a game with the maximum complexity score.
General error 2: Open Frontline(except as a reaction to an open frontline in your USSR home territory)
Leave spots open in your frontline in enemy territory? The enemy will say "thanks" and block your supply, isolate your units and (in the USSR) establish a second line behind you whilst regaining national will. Especially if you are in a disadvantage(e.g. Russian Winter)... try not to leave holes in your frontline. It can be messy really fast. I had multiple USSR 1941-1944 scenarios ending with an allied win in early 1942.
General error 3: Not attacking the air units as axis with air strikes before starting the attack.
Terrible. +2 for you and +2 for enemy isnt good for you(at least not in that situation) because high results are not as favourable for the attacker than low results.
Main game error 1:Bad event timing with Axis.
This error will ruin your game. If you trigger East Invaded or West Invaded at a specific time expect the last policy to fall 13 months later- or whenever you end it. Ending the last policy in July is good. August or September is a MASSIVE advantage for the Western faction later. That's because if you end the policy in July, the game ends four years later. In August, it ends four years and 11 months later. But the USA counters will still come a fixed time after the end of the last policy. Also important: With East Invaded you REALLY dont want to trigger it before necessary. It has the same implication as above, albeit less with units but more with surprise attack markers, factories, and the Partisans marker. Going into Lithuania, Estonia and Latvia(if possible) to push your frontline... no, just no.
Main game error 2: Unnecessary declarations of war
Do you need to declare war to achieve a goal? Go for it. Is it not the right time, you are busy elsewhere, or the benefits are not worth the extra strategic power for Soviet later? Don't. Similarly, as allies, don't declare war against something too early! Especially not Italy(it has an air unit that could help them with the invasion of the UK, Mediterranean or USSR)!
Main game error 3: Bad diplomatic decisions:
-Empty the pot as Allies
The axis players conquers a country, puts a pro-axis marker in a country as well in the pot, then uses that marker to get a guranteed activation of that country...
-Activate a weak, unnecessary country as Axis.
(Latvia, Estonia, Netherlands, Belgium, Denmark, - notably not Iraq in some situations because of the Sea Zone strategic combat bonus, and in very specific situations maybe Syria...)
-Activate a country as Allies except:
--Italy
--Soviet Poland(Wide Open, Appeasement has ended)
--Turkey(in this order, although if you can only activate this one, probably, go for it)
--a useful country if the diplomacy phase is expected to end very soon, especially if it is likely the last
--a useful country during a late Moscow Treaty
Allied Italy is extremely good for allies. It permanently denies Italy for Axis and disrupts all Axis strategies because they have to urgently secure southern Germany from an italian invasion... and because their plans for the Mediterranean are probably dead, freeing up allied units for other spots - like Mainland UK.
Turkey: two reasons. First, Mediterranean. If Axis Italy and then Axis Turkey activate the Mediterranean is probably lost.
Second, USSR doesn't want Axis Turkey either. Yes, it gives them two conditional units but it also gives Axis three full minor country units ready to attack at the southern flank! And they do not cost German production.
--as Soviet, ignore the possibility of an Axis Finland without a ceded area.
That's terrible for later, possibly. Instead of being able to quickly dispatch Finland via that area you might even end up with a conquered Leningrad... which gives Axis a direct USS line into the north and cleans up a potential point of resistance! However, if West Invaded has already been triggered and Norway, Sweden and Finland are neutral countries without an axis marker, you should not try to get Finland.