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Depends a lot on how it ended up succeeding and how the characters feel about the fact that it did:
Achievements in Ignorance, Spanner in the Works, Unintentional Backup Plan, Something We Forgot, Spring Time For Hitler, and Gone Horribly Right are good places to start.
Edited by Scorpion451Thanks, I think Unintentional Backup Plan is the closest.
With a "0", not an "O".It can also result in Sweet and Sour Grapes.
Is there a trope about characters planning something, deciding to abort it midway, only for the plan itself to succeed on its own without their involvement later?