Oh, you're in for a "treat".
When playing Megaman Battle Network 5, I got up ALL THE WAY to the Undernet (that's quite a ways into the game, you know), and at that battle map....well, the game froze while it was saving. So, not sure what else to do, I turned it on, then off, and...the game file was corrupted. I lost EVERYTHING. So, being the hopeful kind of guy I am, I decided to start a new game, anyway. Hey, I could always re-play the game, right? No harm done, right? Well, after playing for a bit, I saved, turned the power off, turned it on later, and...it didn't save! So...Yeah, I took the game back to GameStop. And I never played Megaman Battle Network again...mostly becuase Gamestop doesn't sell GBA games anymore, and it's more convenient buying them from there then online, so...yeah, guess I'm just lazy. That being said, the ONLY WAY I'd ever bother with 5 again is if it was the DS version. Otherwise, NEVER AGAIN.
...Oh, and there was that one glitch at the Slaughterhouse in Beyond Good and Evil that made it impossible to continue if you saved at a certain point. Though, I didn't lose quite as much in that instance, and Beyond Good and Evil was good enough to warrant replaying through all that progress. Seriously, it's ****ing awesome, you should buy it and play through it if you haven't, like, done so already.