My gaming group usually has 6 players – 3 Axis and 3 Allies. As far as we are concerned, it’s up to each player and their allies to catch any mistakes, small or large. Now, we often ask each other about rules clarifications and everyone helps out even if it’s to our own detriment. Also, there are times that even though the next player is starting their turn, the previous player will notice a glaring error and we will let them re-do their NCM. That’s just friendly gaming. However, if they don’t notice it, and neither do their allies, then so be it.
By the way, even what looks like a terrible blunder can still work out in your favor. In one game, Germany (me) was actually preparing for Barbarossa and had decided against Sealion. The UK player, even after losing all the fleet around Britain, did not properly set up defense for London and was trying to rebuild the fleet and place stuff in South Africa to fight the Italians. As I was not preparing for Sealion, I only had 3 transports and my battleship, the cruiser had been sunk against the Brit and French cruisers. I looked at UK and there was only 4 infantry, 1 artillery, 1 fighter, 1 French inf and 1 French fighter with NO ships in SZ 110. I figured UK was asking for it so I gave it a try. I hit UK with 3 loaded transports (3 inf, 3 art), 2 fighters, 3 Stukas and 1 bomber. LONDON WAS MINE!! Right? WRONG!! Here’s what happened. UK didn’t scramble so I had the BB shot, which missed. Then UK AA Gun opened up and killed 1 fighter, 2 Stukas AND my bomber. My inf/art rolled and got SQUAT. UK inf/art rolled and got 4 hits! My fighter and Stuka got hits but so did both UK and French fighters. Result: German land force wiped out and 2/3 Luftwaffe shot down for a loss of 1 Brit inf and 1 French inf. Remaining fighter and Stuka retreated to Holland. THEN, remember that UK was trying to rebuild it’s fleet? Those ships were sitting in SZ 109 and on the UK turn, they came around with the Brit fighter and killed my battleship and transports.
Granted, this may be a “freak” occurance, but I have seen it happen in other situations in other games as well. A player seems to make a colossal blunder leaving something woefully underprotected then when the enemy tries to take advantage, they get smacked down.
Oh yeah, one other thing. Sometimes a player will deliberately leave what appears to be a mistake (high IPC territory underdefended, expensive units unprotected, etc.) yet it will be a trap. The enemy player will swoop in to take that territory or destroy those units and they themselves get pounced on. Maybe even lose their capital. So “mistakes” aren’t always what they seem.