I've read a probable scenario about accidents with small DSVs in the book Below(Edit) the Edge of Darkness (about marine biology of bioluminescence and the tech developed by the author to film/record it):
There was a small leak in a valve, used for emergency operation IIRC and the author noticed that going down - she was still light enough to ascend but she said had she been a few hundred meters deeper, she never would have made it back up due to the extra weight.