backtrack

To allow blood to flow back into the needle during drug injection (with heroin, for instance).

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'backtracking' is equally often used in a metaphoric sense, or one not connected with the literal meaning given by Marvinius, of retracing your steps, going back to the last junction and trying a different route.

You may well have to backtrack if your changes to a computer program, for example, do not have the required result; reload your backup (you did have a backup?), and try again. Similar to Back to Square One, but perhaps requiring additional work to get back to that pristine state.