 At one point during the return flight there is a bang and nobody is  very alarmed; its just a "burst helium disk." This was actually a  significant event, though an expected consequence of the situation.  The helium disk served a protective function in the LM descent  engine and, after it burst, they might no longer be able to restart  that engine. A final course correction, not shown in the movie, had  to be done using thrusters instead.