Ok, the generic name for the drive gear on a starter is the bendix, just like slip joint pliers is usually called Channel Locks, in the South a soda is a Coke, etc, etc, etc.
That said, some starters have a one way clutch, sprag, roller, or otherwise, and some don't. In any case, even a one way clutch can lock up and not allow a reverse rotation if it gets dry, is spun too fast, etc. This being the case, as I stated, the engine can easily turn the starter.
Having been around and worked on machines for the majority of my 46 years, I know of others that have seen it happen, have seen the results of it happening in starters at the repair shop I use, and have seen it once for myself that I can recall on a piece of heavy equipment and several times on small engines.
In my personal experience the heavy equipment incident went like this. A call came in about a CAT scraper making a strange noise. When at an idle you couldn't hear much, but when you wound the engine up you got a high pitch whine that sounded like a turbo, but wasn't. When I got there and started looking, got the operator to run the machine around while I listened, etc, we finally traced the sound to the starter. By that time the starter was so hot it took two pairs of heavy welding gloves on my hands, and another pair as a buffer, before I could comfortably put my hands on it to do anything with it. Once out of the machine, it was obvious what had happened as the drive gear was stuck in the extended position. As you'd expect, this kept it engaged with the starter ring on the flywheel.
From that point, it was several hours before I got it back to town to the repair shop. At that time I still had to wear one of the pairs of gloves to pick it up as it was still that hot.
When they opened it up the windings were screwed up, the bearings were blue from the heat, and the thing was basically so close to locking up/blowing up, it wasn't funny.
That all said, a common number of teeth that I can remember on a drive gear is 13, and a number I can remember counting on a ring gear on a piece of equipment is something like 140. This gives approximately a 10.769 to 1 ratio. That being the case, if a starter and ring gear with this combination locked up, when the engine was turning 2800 RPM, (high end for many diesels) the starter would be turning in excess of 30,000 RPM. This is about the speed that many turbos turn, thus the whining sound heard when the starter on the scraper hung up.
Doubt it if you want, but having been in the industry all my life I have seen it happen at least once on a piece of heavy equipment, and several times on small engines myself. Between personal experience and the experiences of others I have heard of or seen second hand, there is no doubt in my mind how ugly, and expensive, the result is.