What should the congregants do if the imam stands for an extra rak'ah (a fifth in a four-rak'ah prayer) or sits in a deficient rak'ah (the third in a four-rak'ah prayer), and does not respond to their tasbeeh?

The Imam must return from standing for the extra rak'ah if alerted by two reliable men. If he does not return, then it is not permissible for the congregants to follow him in the increase. Rather, they may separate from him and perform the taslim, and their prayer will be valid. However, if they follow him out of ignorance that this is forbidden, then their prayer is valid.