A slip leads to a horror striking the head (5)

I believe the answer is:

error

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'a slip' is the definition.
(slip can mean an error or mistake)

I don't understand the rest of the clue.

(Other definitions for error that I've seen before include "Miscarriage of justice" , "A mistake or inaccuracy" , "Slip-up" , "Gaffe" , "Miscalculation" .)

I've seen this clue in The Irish Times.

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