A murder/death has taken place and the investigators find what looks like a part of a name written in blood, or scratched into a nearby surface, or written in condensation on a window (whatever is to hand).
The entire episode proceeds with the assumption that the "name" has been read/understood correctly, but it is later found to have been misinterpreted.
A recent episode of Lewis saw "500" scratched into the metal of a car by the victim. The detectives automatically assumed this was a telephone extension number and proceeded as if this were gospel. As it turned out, it was really a Chinese name "Soo Lin" or similar.
I think that this is only a trope if the audience immediately thinks "well, maybe it means something else" but it takes the _characters_ the rest of the episode to find out (like Tales of The Unexpected).
There was an episode of Jonathan Creek where somebody was predicted to die "at the time of ISIS". It turned out that this was really "15:15" on a digital clock (Americans wouldn't immediately get that, of course). The digital clock display was shown prominently several times until the reveal; it was obvious (to me, anyway) what ISIS meant.
An episode of B7 had a murder victim scrawl "54124" in blood. The episode proceeded with people trying to work out what the number meant. The actual blood writing was not shewn until later, when its meaning became obvious to the audience, but not to the characters.