They almost trashed the adventure
A few episodes ago I confronted my crew with a semi-typical "Lava creature", which was actually a walking history bank on the lost civilization of the planet. Well, maybe I've run too many horror games, but here's how the scene played out. (Remember, Starfleet protocol is not to fire first.
Me: *describes Lava creature*
Captain: "Gah phasers on 12! Fire!"
*crew responds and fires, some set their phasers to 13 "just to me sure.*
Me: 0_0
Captain: "Did we hit it?"
I think explaining to them they they brought half the cave they were in down upon them got the message across not to hack their way through problems in the furute. ;)