“Snug it up Buttercup”, is what I tell people in the gym when they put on their harnesses. Here’s an example of can happen to a non-snugger-upper. (BTW, she doesn’t look all that excited about jumping out of the plane in the first place.) More of her story here.
“A US woman's 80th birthday nearly ended in tragedy when she slipped out of her harness during a parachute jump. Laverne Everett decided to mark her birthday by fulfilling her longheld ambition to go sky-diving. But once in the plane, above Lodi, California,she found she was no longer so keen to jump. Her instructor, jumping in tandem with her, freed her hands from the doorway and appeared to almost force her out of the plane. But something went terribly wrong. Mrs Everett nearly fell out of the harness, and her partner had to hang on to her for dear life.”