Moving the goalposts is the fallacy of changing the claim (along with its requirements for a disproof) upon seeing one's claim about to be proven false.
The phrase of "moving the goalpost" may relate to an analogy of a racer about to reach the finish line, only to find that the goalposts marking the finish line have been moved as the racer comes close to crossing it.
- Theist: I prayed to God to heal me when I was sick, then I got better, proving that God exists.
- Skeptic: But the human body has the ability to heal itself; how is your recovery from illness proof of God's existence?
- Theist: Who else but God would have made the human body with the ability to heal?
In this exchange, the theist is changing his claim of proof of God's existence from "God healed me when I was sick" to "God gave made me with the ability to heal myself" after hearing the disproof of his first claim by the skeptic. In doing this, the theist challenges the skeptic to disprove this new claim while acting as if it were the same as the first.
- Creationist: No one has ever seen a species change to another species.
- Evolutionist: Experiments in the lab have produced new species of fruit fly. This has also been observed in the wild.
- Creationist: But they're still fruit flies.