Has anybody else ever had people show up to your convention games expecting to be playing a completely different game based on a sloppy reading of the description?
I had a group expecting Apocalypse World because I used the word Apocalypse in the scenario name even though the game description was clearly something else.
I had someone expecting 5 Torches Deep come to a game of Torchbearer.
There was another time that GURPS Transhuman Space got mixed up with Eclipse Phase.
These sessions were by far the worst convention games I've ever been in. The players quickly disengaged, dicked about, fell asleep or left the game when it wasn't what they thought it was.
How can a GM stop this from happening? Or if it does what can you do if this is clearly happening at your table?
No, but this is a common experience across settings. I've seen people mistake addresses, interview dates, job descriptions, meeting purposes, operating instructions, basic signage - basically everything, told or read. Unless you're making regular, personal contact beforehand to make sure everyone's on the same page you can't really avoid it.