Three times, all in the Gulf of Mexico.
1. Giant strided from the MV Fling (high dive gate) and descended only to find my mask flooded a bit. After repeated clearing, I got a bit exasperated and gave a pretty good puff...and blew my mask right off my face! When I hit the water, the strap rode up onver my head and only water pressure was holding it on. I caught the mask before it got away, donned and cleared it, and went upon my merry way.
2. While hanging on the mooring line, I turned my head and the current shoved my mask around my head. Yup, the current was really a-rippin' that day. It was only dislodged, so I don't know if this counts as "losing" my mask or not.
3. While towing a diver along the tagline, I looked back to monitor his condition. Just as I turned to look forward again, I encountered a short line to a float attached to the tagline, and it dislodged and flooded my mask. Bad timing...I had to clear my mask before I could see to continue the rescue. Took about three seconds, but that seems like a loooong time in those conditions.