Letting batteries drain too far:
- In phones where it's hard to replace and old laptops where it's hard to source a replacement
- In AA/AAA-powered devices where a dead battery will evolve hydrogen gas and eventually break its seal, causing a leak
Also, I really should be buying Lithium AA/AAA cells now that I have so few devices that need them, but I keep buying a pack of alkalines each year. Ni-MH voltage falls just a tad short.