Valve stem seals, piston rings, PCV valve, breather filter, incorrect emissions line connection (did that one once on a 5.0, kept fouling one plug due to the PCV valve's vacuum hose being hooked up to the intake wrong) cracked head...
If you want to keep the car on the road, you can get a set of non-foulers from the "HELP!" section of any parts store to keep that cylinder from mis-firing, but the oil sludge will still build up in that cylinder and eventually cause more problems.
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
Shall it rest in peace. 
