Two I can think of...
I used to believe that all who go through the Great tribulation (who have missed the rapture) have lost any chance to be saved. Basically, left behind and doomed to hell. This was after watching an end times movie, and before I read a bible. It stayed in my head and I assumed it was true until I actually got to studying the end times for myself. I was surprised to learn there are some out there who believe this, even after reading the scriptures.
I used to believe the "falling away" in 2 Thess 2:3 was speaking of the rapture of the church