Split JD 7800 for a roll pin in the trans shifter.

Rockshaft on 4430 cause factory couldn't bend the cotter key.

Pull PTO on 1456/1486 cause the u-bolt on depth shaft let the arm move and fell off draft linkage.  FWIW if your arm is real small you can do this thru the top link hole.

Re-split 1456 cause we (all three of us) forgot to put seal in input shaft.

Pull engine on 8N, forgot to put one main bearing shell in.

Pull heads of 392 IH V-8 because the dealer gave us the wrong gaskets.  They were cut about 1/8" shorter, covered the hole in the block but the hole in the head was bigger.  Learned to double check them, to this day I save the old gaskets to compare against the new ones!

If OP had asked, in general, hard to work on, then darn near anything on the 815 combine, especially the spark plugs on the bank against the grain tank.  Oh wait, even better, the points on Massey 92 combine (I think that was the model) that had the engine on the bottom.