The Ad Hoc Rescue fallacy is when an arguer, to defend their argument from a counterargument, creates an unsubstantiated assumption to help explain away the counterargument.(IEP, Dowden)
Example:(IEP)
Yolanda: If you take four of these tablets of vitamin C every day, you will never get a cold.
Juanita: I tried that last year for several months, and still got a cold.
Yolanda: Did you take the tablets every day?
Juanita: Yes.
Yolanda: Well, I’ll bet you bought some bad tablets.