The first time I remember absolutely losing my suspension of disbelief was at the end of the first Mission Impossible reboot where Tom Cruise puts an explosive on a helicopter he's hanging on the outside of that's flying behind a train through a tunnel, and the explosion completely destroys the helicopter and flings him onto the back of the train. Yeah, that helicopter (which probably couldn't be flying through a train tunnel to begin with) was made of far tougher material than Tom Cruise. Any explosion that destroyed it, would have turned him into a stain on the wall of the tunnel.
If a future sequel revealed that Cruise was, in fact, turned into a stain on the wall of the tunnel and an alien has been masquerading as him ever since, it would be less ludicrous to me than Jim Phelps turning out to be a mole. I read that Peter Graves, who played Phelps in the TV series, turned down an offer to reprise his role when he learned of that plot twist.