[just another ref]

Originally Posted by Nevadaref

10.5.3 SITUATION: A5 has just received his/her fifth foul of the game. A5 (a) is
erroneously permitted to remain in the game for another two minutes before the
scorer realizes the mistake; or (b) leaves the game after the coach is notified of
the disqualification. At the intermission between the third and fourth quarter, A5
reports as a substitute and subsequently enters the game. RULING: In (a), as
soon as the error is discovered, the player is removed from the game, no penalties
are assessed. In (b), A5 will not actually "participate" until the ball becomes
live. If detected prior to the ball becoming live, A5 would be directed to the bench
and no penalty assessed unless the official deemed it was a deliberate attempt to
circumvent the rules. If detected after the ball becomes live, it is a technical foul
charged directly to the head coach. The player is immediately removed from the
game and Team B is awarded two free throws and the ball. (2-11-5 Note 2)

What about the sub who has already checked in? Sounds to me like there were 6 on the floor. If so, this case doesn't fit the OP.