Here is a video from the Irish Farm Journal that interviews a farmer who lost his left leg and severely injured his right leg in a PTO shaft entanglement. PTO shaft injuries are one of the top causes of ag injuries and fatalities after tractor rollover and on-road collisions. The main two factors leading to this injury were loose clothing (his overalls broke and he let one of the straps dangling) and working near a PTO shaft while it was running. Just as a warning some of the descriptions of the incident are pretty graphic and they show what his legs look like now.
I’m also sharing a link to the Pennsylvania Extension’s guide to preventing PTO injuries and to a place that sells easy-access replacement guards that can be pulled back for easier access to grease fittings. Some states have programs that provide free/reduced-cost guards, so be sure to check with your local extension office to see what is available in your area!