I had a Gamecube with about 13-14 PAL games and 3 NTSC games. One day my younger brother decided he felt like playing Mario Tennis (NTSC) not realising that he also needed to change the memory card. The game warns you about having to reformat the card but my brother obviously didn't read it.
Gone were completed save files for:
Mario Sunshine
Smash Brothers
Star Wars
Waverace
Eternal Darkness
Pikmin
Tiger Woods
Starfox Adventures
Zelda Windwaker (plus Ocarina of Time and Master Quest)
Metroid 1 + 2
etc
What's more he played Mario Tennis for about 10 minutes and decided he didn't like it.







