T or nothing

[just another ref]

I believe there is a case play that addresses this which states that if this happens during a substitution involving multiple players and is apparently a result of confusion and no advantage is gained, play on.

[PG_Ref]

This applies if it is not after a timeout or intermission...

Caseplay
10.3.2 SITUATION B:

After a lengthy substitution process involving multiple substitutions for both Team A and Team B, A5 goes to the bench and remains there, mistakenly believing he/she has been replaced. The ball is put in play even though Team A has only four players on the court. Team A is bringing the ball into A's frontcourt when the coach of Team A realizes they have only four players. The coach yells for A5 to return and he/she sprints directly onto the court and catches up with the play.

RULING: No technical foul is charged to A5. A5's return to the court was not deceitful, nor did it provide A5 an unfair positioning advantage on the court.