Utter inability to assess their chances.
The situation: Exploring a subterranean lair filled with illithids and enslaved githyanki thralls. No hurry, they could have rested / retreated anytime.
The party: Cleric, Fighter/Bbn, Sorc, Ftr/Swashbuckler/Rog. All bases covered.
Room 1: The party defeats some thralls. The cleric has to use half his healing spells, though. Fighter was down to 1 hp, got healed.
Room 2: The party enters another room and defeats the thralls there, including an illithid. The cleric and sorceror are almost out of spells, the Ftr/Bbn is - once again - down to one hit point, the rogue to 1/2 his hp.
W.W.A.R.P.D. ? (What would a responsible party do?)
"Let's see where this next door leads to."
As it turned out, it led to a pair of spiked-chain-wielding githyanki. 2 out of 4 characters kicked the bucket.