When a proper appeal is made, what condition can lead to a run being disallowed?

When a proper appeal is made, a run can be disallowed if a base runner misses a base. This is because missing a base is a fundamental infraction in baseball rules that can be subject to appeal by the defensive team. When the defense successfully appeals that a runner failed to touch a base, it results in the runner being declared out. If this occurs after a run has crossed the plate but before the appeal is accounted for, that run would not count, since baserunners are required to legally touch all bases in the correct order for the run to be valid.

In contrast, if a runner is out for interference, the situation may lead to different outcomes, including the possibility of all runners being returned to their original bases, but it does not inherently disallow a run unless it directly impacts a runner's ability to score. Similarly, a declared safe call for a runner does not affect the validity of a run scored, and a strikeout for the batter does not contribute to the conditions that invalidate a run. Therefore, the condition of missing a base is the specific situation in which a run can be disallowed following a proper appeal.