Don't you just hate those times when a game that was such a cakewalk before decides to sucker punch you with a sudden insanely difficult and frustrating boss battle that takes countless retries before you get past it? Because I love it. *cough*sarcasm*cough*

I've just replayed Donald Duck: Goin' Quackers for the PlayStation 2, which was even easier than the Nintendo 64 version which I reviewed earlier on Vizzed (well duh, it's a kid's game, I don't know why I even bother with them). The game is designed in such a way that you automatically farm lives as you casually play through the game, and I hardly ever lost lives, too. So before the final boss, I had about 70 lives. Holy cow, that's a lot. And then I started the boss battle, and lost half of them. Around 35 lives. I had NO idea what was going on. The game had been so easy until then.

Anyway, do you have any experience with these kinda situations? What's a boss battle that comes to your mind?

when you have to fight Bowser for the last time in Super Mario 64 when he breaks the platform and turns it into a star
its really hard to throw him into another spike ball well for me anyway  

Mine was the Chaos Chimera from Golden Sun: Dark Dawn that boss was a huge difficulty spike not only does it do a lot of damage it moves very quickly and has more HP then I can comprehend. My characters are all in the high 70's with their strongest weapons and armor in the game and I still get murdered by that boss. It is the only enemy that managed to defeat me in that game and that is saying a lot. It is so hard that I got the game back in 2010 and still have not beat him so yeah if you ever play that game be prepared for that boss. By the way it is a story boss so you have to defeat it to beat the game so yeah.

bass player 9 : There are 2 games I can think of off the top of my head. One is the Rachet and Clank Games. The game play up to the bosses isn't too bad, but once you get to the boss it takes forever to defeat them and quite a bit of retries. The other game is Resident Evil 5. The fact that you are unable to move while shooting makes everything much more difficult. Especially when the random zombie with the chainsaw comes after you. 

I know it's not a boss per-se, but I Always considered the giant characters from DBZ budokai tenkaichi 3 to be bosses, the saiyan saga was ridicoulosly easy, but then, when you had to fight Great Ape Vegeta, it was really difficult

Oh, you mean pretty much every Kirby game ever with every single boss? Seriously, the Kirby games are never all that hard until you get to the boss, and then you find yourself getting your butt kicked every time (Kirby's Dream Land 2 is especially bad for that, I find).
I think another prominent one I can think of is Huff N' Puff from Paper Mario. Every other boss (Even the final one) is easier than that guy, since he gets stronger the more you damage him, and he's one of few bosses that can heal. Granted, he's not too bad once you realize Lakilester is pretty much your saviour there, but until then he sucks to fight against.