So last night I did a compression test on my F500 motor (two stroke twin) and the cranking pressure was 45psi and 60psi (normal is 70psi). At the last autocross the motor was very reluctant to refire between runs.
After measuring the cranking pressure I was left with "how the hell was that thing still running". I'm sure lots of you have had similar experiences.
My first 5.0 Mustang bought used for cheap. It was consuming oil (leaking it) at a pretty high rate. When we did the tear down the front main bearing was paper thin. It still ran quite well even with bad mains all around.
I spun the #1 bearing on my truck and still drove it for about 6k miles or more before I could afford to complete the spare engine.
I bought a Holden in new zeland that seemed to run pretty well. It would foul a plug in 600 miles and had about 60 psi in one cylinder but with a plug change would run great. Drove it for a month there and sold it for more than I paid for it.
Broke the ring lands in #3 in my Volvo 850T.
It ran ok as long as I was on boost.
Made it back about 20k to work because that's the shop I was closest to. Didn't want to run after that.
Raced this XJ in a few different cheap truck style off-road events and this was the result. Something inside the engine busted a hole in the block and the oil pan yet the engine continued to run with seemingly no issues.
Not sure if this counts, but my parents had a 2003 Cadillac STS that severely overheated at least 3 times but never due to headgasket failure. First, due to a broken radiator, next a split coolant hose, and finally, a failed water pump. They got it fixed each time, and the engine continued on no worse for wear.
When the car finally died at 190k due to a suspension failure, the engine still ran flawlessly.
The balancer cracked on the 302 in my panther LTD. Unfortunately, I couldn't feel the vibration due to a bad transmission mount. The vibration wore the end of the crankshaft enough the the replacement balancer was loose.
When we pulled the engine to rebuild it the cylinders had about a 0.10 ridge at the tops of the cylinders, but it had been consuming no oil between changes.
When I overheated my Subaru, the gauge had been pegged for about five solid minutes of driving at WOT. The car's top speed went up from 87 to 92mph! Must have been a combination of reduced parasitic drag from the water pump not having to pump water, and increased thermal efficiency from not losing as much combustion heat to the cooling system 
When I got to work, it stalled in the parking lot. It had no compression and would not restart.
Later, I put water in it to see if I'd hurt the engine. Cranking the engine shot muddy looking water about six feet out the grille. Yes, I'd hurt the engine.
The incident where I learned the value of proper air filtration on a rotary, the side seals wore grooves in the side housings .025" deep. Maximum wear is .004". The apex seals were worn to 1.6mm thick, too, which is not a dimension that they normally wear. It still "ran" but by the time I got home, you could floor it in 3rd gear at 3000rpm and it would do nothing but get louder.
I bought a Morris Minor Travelar ( the woodie Wagon ) for $50 the owner sold it because the inner fender had some surface rust and he convinced himself the suspension was going to collapse.
At that price he kept the battery. So my buddy pulled me home with his truck. Rope no less. Brakes didn't work either. So I let the clutch out in gear. Just for grins I turned the key on before we left. A few stop signs later I noticed every time I let the clutch out the dome light came on. ( remember, no battery ) then one stop sign the engine was running. Nice and smooth. But uh, no brakes? Thump,••• I ran into his truck but the engine kept running. I pulled the parking brake on and ran up to him apologizing for bumping into him but excited because the engine was running without a battery.
In reply to frenchyd :
Most cars of that era were capable of that trick ...
None of the young guys on here that were brought up in the alternator era (sarc) would have encountered it but there was enough cast iron and residual magnetism in the old generators that they didn't need a battery to excite them enough to produce electricity.
I've got one that I still can't figure out. A while back my truck got rear ended in heavy LA traffic by a guy driving a late 90's Maxima. He hit me pretty good, and being lower, his bumper went under mine. The headlights/grill/hood got destroyed, his valve cover even took a hit. We were stopped in the middle of the freeway and the CHP officer told him to drive to the side of the road. I looked at the sheared off battery and thought "good luck with that." It was just the terminals and a bit of the lid dangling from the cables. But it fired right up and drove to the shoulder!?!
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