Door cards are usually not marked with room numbers these days, a lot don't even have the hotel name on them to prevent unauthorised access.
But messy writing on the cardboard folder is a problem sometimes. If there is a lineup well trained staff dont yell "You are in room 307", they just say your room number is here...and point to it.
I had 3 goes to get the correct room at one place. The writing was messy and I thought it was 209, nope, wont open, maybe 409, lady inside told me no it was occupied after hearing me at the door. Back to reception...609 it was apparently, got in, room made up properly etc, so dumped bag and used the facilities (in a hurry by now) as above and then noticed someone else's clothes and things hanging about. Back down again, more apologies, upgrade to top floor unoccupied room at last...
Think it happens a lot, though hopefully not at 4am.