You are laying to two anchors because the anchorage is crowded, It's time to leave, the tide is low and a fresh wind is setting your boat back on the main anchor which means there is lots of slack in the stern anchor rode. In the excitement of setting sail, you start the engine, put it in gear forgetting the stern anchor because, if you are like most of us, you rarely use it. The engine stops. Your anchor line is around the prop. It happened to us on a delivery trip and taught us two tricks.
1. Now, every time we are on a boat with an engine, if we set a stern anchor, we secure it then clove hitch the anchor line around the gear shift lever. That way we can't start the engine without being reminded to keep the slack out of the stern line.
2. We were able to save the stern line on that delivery and get it free without going into the water. We loosened off the bolts on the propeller shaft coupling, then levered the shaft back just 1/2 an inch. Then we p8ut the inboard end of the anchor rode on the sheet winch and pulled slowly. It unwound itself with only some bottom paint showing where it had been wrapped around the shaft. Then the bolts we re-tightened and my skipper was, as the British say, "Happy as Larry."