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An action taken to prevent an undesirable situation is called a prophylactic, a proactive or a preventative action.
An antigoal is an undesirable situation, which one should be keen to avoid.
Pickle
A banjax is a mess or undesirable situation made as a result of incompetence.
Figuratively, a camel's nose is a metaphor for a situation where the allowance of a small act will lead to a larger, undesirable act.