"My home base has farm fields north and east adjacent to the airport, big and beautiful year round. One late afternoon hot day (calm winds) I was riding along in the right front seat of an older and lower horsepower Mooney (M20C), with another pilot seated in back. The three of us were headed out to an airport restaurant for dinner. Taking off on runway 31 (4100' long), I thought the runway roll felt a little slow, but everything else felt and sounded normal. Figured it was a hot day and we had 3 adults on board (one big guy), so I wasn't expecting normal performance.
Just after lift off, it wasn't climbing as much as I expected, but the owner pilot didn't seemed concerned as he quickly retracted the gear with the Johnson Bar. Only then did the three of us start talking (paraphrasing), "hey, this doesn't seem right, we're barely climbing. Let's go back and land".
During our conversation we passed over the end of the runway, over a busy road, at no more than 100' or 200' feet high, tops. But it was still climbing, barely, with no vibrations, and sounding completely normal.
As the owner began talking to the tower to announce a return I was sticking my left hand in his windshield with my thumb pointed right, and motioning to the right. Which would have kept the aircraft over nothing but farms on downwind, right base, and final, then landing onto runway 26. By keeping the downwind tight the engine could have quit and a right turn would be on the airport, somewhere. But, the owner wanted back quickly, and turned left to land on runway 8.
During the left turn there was nothing but houses and neighborhoods, and a return over the busy two lane highway with rush hour traffic backed up. Not exactly my choice, but thankfully the engine kept running.
Turned out one cam lobe was completely gone, and the engine went right out for a rebuild at around 1000 hours from the last rebuild."