[DaveNP]

Lawn Mower Syndrome
It shouldn't be a car killer and quite how common it is I don't know (except I've had it happen to an 850 and a V70)
The basic scenario is that a car that hasn't had any other problems is started for a short time, such as moving it out of the garage to get the lawnmower out from behind it, and then stopped. Next time you go to start the car it spins over OK but won't fire up. There's a couple of theories on the actual cause, I tend towards the one that the hydraulic tappets on older carts get a bit crudded up and then the cold oil doesn't flow so well leaving the valves slightly open which means there's no compression. The way to avoid it is to be sure the engine is warm before you turn it off again. When it happened to me I pulled all the plugs, put a squirt of oil in each cylinder, turned it over a couple of time with the plugs out , put the plugs back and it then fired up in a cloud of blue smoke.

Snot on the dipstick
The engine tends to have a lot of blow by gases in the crankcase, if the rubber rings on the dipstick have hardened and don't give a good seal the gases try to escape up the dipstick tube, when they condense in the tube they leave a load of oil/water emulsion snot on the dipstick. it can lead people to think the head gasket has blown but changing the rubber O ring on the dipstick does the job* and maybe sorting out the Positive Crankcase Ventilation PCV system to allow the gasses to be sucked back up to the manifold and burnt off in the cylinders.

*a new dipstick and tube isn't too expensive from Volvo.