These are phrases that stop you from doing something 
stupid, or at least alert you that there is a problem in a positive way.
Unlike KillerPhrases or AlarmBellPhrases, the person uttering them is 
aware rather than ignorant of the open manhole in front of him.
Usually they stop work/effort/discussion in it's tracks, but it's a
good thing because you entered the time-wasting zone.

That probably wasn't very clear, how 'bout some samples instead:

"It's 2AM"  - implies everyone is in the veggie zone, and it's pointless to continue trying to come up with 'good' ideas, silly is all that's left.

"NO KILL I" - points out that the text under scrutiny is too ambiguous 
for any meaningful interpretations.

"Where is your test?" - When PairProgramming and about to add functionality.

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What about a TitForTat page, with a list of pairs (KillerPhrases ,AntiKillerPhrases)? -- FridemarPache