Outside of dead batteries, by far the most common issue I've run into replacing li-ion batteries is swelling. Besides causing physical problems inside of devices (dislocated screens, trackpad malfunctions, etc.), they are a hazard to deal with and require special handling.
This is a must read for lithium batteries:
[[What to do with a swollen battery|What to do with a swollen battery - iFixit|new_window=true]]