The most common error that I see, and it causes most of the problems, is trying to rebuild a Crosley engine without reading the service manual first.
Another problem which leads to many failures or less than satisfactory results is, not inspecting and properly measuring the parts and reusing pieces that should be replaced. Just stuffing a set of rings in worn or gouged cylinders will NOT make much of a rebuilt engine. Many parts need to be replaced or reconditioned to do a decent engine job.
Not going through the oil pump properly (or even knowing what to look for, to determine what it needs) is another common error that we see.
Then as Barry Seel has said, we have NEVER found a Crosley engine that had 8 good , serviceable valve guides. Usually there might be one or two, but generally they are all worn, and the seats are recessed and pitted to the point where you can't get a good seating surface and still maintain any spring pressure. (The valve springs are throw away items also.)
The most FATAL error is setting up the lower towershaft/crank bevel gear set. Too tight, you break the crankshaft; too loose, you wear and strip the teeth. We just finished a repair on the later, and I have several cranks with broken noses due to the former.
It's not rocket science, I've built a lot of Crosley engines, and the worst thing that I've done (on a completed engine) was miss the camshaft timing by a tooth. (This doesn't count the pile of scrapped blocks that I made while getting the machining part figured out, like listening to a customer and boring a block .179 thousandths oversize, IT WON"T GO THAT BIG, trust me!)
I taught myself, both the building and the machining (but I had built a lot of engines before, and tore down quite few Crosley engines before I started rebuilding any), I figured out the procedures (that work and those that don't) and built the tooling and jigs that couldn't be purchased to do it. That's my nickels worth, sorry I got so long winded.