Not straight up stealing your work, but some contests or things like mentorship programs etc. will have you submit a script and in the fine print you'll see that part of submitting is they have the right (or option) on the script for 6 months/a year or some shyt.
I was gonna enter a writer's scheme a year ago (forgot the name) and I saw that in the fine print if I made it to the last 50, they'd have rights to my script for a year without having to pay me.
I'd get paid if it was sold, but normally an option like that comes with money. They just straight up would have those rights for free and I wouldn't be able to sell it myself because I made it on their "shortlist"
Always read the terms top to bottom.