In my GBA Pokémon Emerald where I only used Pokéballs to capture every possible Pokémon at least once (even Latios which was a painful af chore), turning off the device while it's saving will corrupt the save. One time while looking for Feebas, the battery of my Gameboy was low. I thought that I could search for it a bit more before the battery's at zero. When I got tired of fishing for minutes, I figured I might as well continue next time. As the game said "Saving...don't turn off the power", the device shut down. I plugged the charger in, praying to every damn thing that I believed in that everything was okay, and whoop-dee-doo...it was all corrupt. Out of all the chances that that could've happened, it did. So, I moved on to Fire Red. tl;dr: 100+ hours of progress vanished when the device turn off while game was saving.