If you're attacking ships below your range from a planet, if you go into a "Use Artifacts" screen it will let you use the artifact, but when you close the artifact box and try to attack the player it says they're out of your range, even though they're still guarding the planet.

Actually, this also seems to apply to truce effects on planet guards. It seems that when you look at the artifact screen the game forgets that you're targeting the ship as a planet guard.