First headgasket I did - went to town on the deck with a little 3m scrubby. Low mileage motor without any issues (was swapping turbo, required head off thus the headgasket). Everything went back together great until a few thousand miles later. Rod knock appeared and my turbo crapped the bed about the same time. Looking at the bearings afterward it was obvious something had gotten in the oil. Despite my efforts to plug the openings on the engine's deck, it wasn't enough. Did some searching/reading, and sure enough, the 'real' 3m pads are impregnated with alum oxide. Anything that came in contact with or pumped fluids in that engine was hosed. The little particles embedded in the bearing material and slowly ate away on the steel surfaces.
Was a tough lesson for a broke high school student.