On the interstate some exits say "Exit Only." I used to believe that meant there was no entrance ramp back on to the interstate. If one wanted to return to the interstate s/he would have to go to the next entrance/exit. In other words, it was only an exit.
A childhood belief about road signs

others believed things like this
- Anyone else think that Emergency Exits were special exits for some celebrity called 'Emer Gency' to use?— Mark Yardley
- When I was young, I really used to believe that 'R.I.P.' (on a gravestone) meant 'Return If Possible.'— Colin
- I used to think that after you passed an off-ramp while driving on the freeeway, it disappeared.— Anonymous
- When I first heard the words "flea market", I actually thought it meant "selling fleas".Anon
- I Used To Believe In The Scab Fairy, That She Would Pay You In Return For Those Scabs!— KellGirl
- I used to believe that "gluten free" meant you could eat as much as you wanted without getting full or fat— Jess