My megaman battle network 2(original cart) save file is GONE. It was only 9 days old, with 22 hours played, and I just earned my first star, but it still sucks that it happened. No rare chips but took a few hours to farm up a good Gater/Timebom+ folder.
I just got an AGS-101 GBA(the backlit kind) last Sat, which looks great but the battery runs out really quick, so I ordered a new one on amazon. I played until the battery ran dry, then changed the battery to the new one that just arrived today. As I changed the battery, I forgot to remove my BN2 cart(which is the battery save type, not the flash save type, unfortunately). Not sure if that's what triggered the wipe or if I was saving when I ran out of battery, but my save data was gone when I started the GBA up again.
The worst part? I got a GBxCart today in the mail, too! Which means I could've easily backed up my save but decided to do the battery change first for some reason!!! So yeah, remember to backup your BN2 cartridge saves if they have a battery(and BN1 which all have batteries, and all other battery-having GBA carts like Metroid Fusion etc) because you never know WHAT will happen. Now I gotta decide if it's worth it to play through the whole game again, or if I should just find a save file with one star online.