1 Eleven: So what do we need to do to get through this Chapel of Evil?
2 Dustin: On the wall opposite the arch there’s a magical stone block that can’t be detected by any magical means.
3 Dustin: It has a small slot detectable only on a 4 in 6 chance by direct physical inspection. It looks like it would accept a coin.
4 Eleven: So what happens if we put a coin in it?
4 Dustin: You fall victim to one of the classic blunders!

It's really impressive how many magical things in the original Tomb of Horrors adventure can't be detected by magic, can't be affected by magic, and can't be circumvented by magic. This is mostly due to its design nature as a cut-throat tournament module, where they didn't want players using sneaky tricks to simply avoid the puzzly traps.