1 Dustin: The mummy preparation room. We need to recover the gold key.
1 Eleven: Sounds easy.
2 Dustin: Half the key is in that vat of acid. And the other half is in that vat, under an ochre jelly that is coloured grey.
3 Dustin: Careless adventurers could easily mistake it for a grey ooze and try lightning bolting it, since they’re vulnerable to lightning.
4 Dustin: But ochre jellies just split into multiple jellies! Making them harder to fight!
4 Lucas: Do not ask how we know this...

This is a hilarious trap for the players who read and memorise the Monster Manual.

Which back in Gary Gygax's day was pretty much all of them.

One could almost accuse Gygax of writing the Monster Manual to deliberately mislead players.