None.
I still remember Battletoads. I beat it.
I also beat I wanna Be The Guy (on just hard difficulty though).
If there is a seemingly unbeatable portion, puzzle or opponent, it simply means that I am playing too dumb to beat it or that I haven't taken everything into account.
Unless it is a clear coding error, everything can be beaten. It is simply a matter of finding out how and trying hard enough. This is also why I hate hard-coded losses in battle. If the loss comes trough clearly way-beyond-my-league hit points and attacks or subterfuge and deception from the enemy (such as BG II's recapture by Irenicus "A spell component in a soup or hidden in a seasoning of a fish, all trough your journey accross the sea. Undetectable and perfectly harmless until a single word from me. There will be no final battle, no heroics. Simply - sleep.") I can accept that as a masterfully executed plot-trist or a Xanatos' Gambit.. But if I am fated, hard-coded to loose a battle no matter what I do, that I hate. Because even if the enemy is supremely overpowered at that moment, if you can find a combo of items or some series of moves and techniques that would allow you to defeat that enemy it should be allowed to be defeated.
Because if the creators of a game put me up against enemies that I might be able to defeat trough legitimately found or acquired items/spells etc and yet preordains my defeat beyond a shadow of a doubt, that is just poor design and lazy storywriting.
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