Well, maybe my tranny moments weren't boneheaded, although I do have a true bonehead anecdote. In my case I was mixing and matching some good bits out of my PV'a M40 with my juunkyard M41. I didn't notice the ever so slight differences in some of the componenets, and thus had repeated problems when trying to get the layshaft back in place. I did that over and over (got pretty good at it after a while) before slowly coming to the realization that some of those 'identical' parts weren't really identical. In the end the only parts that could be swapped were the syncro cones. Luckily, that's all it *really* needed - the M41's slightly more worn gears were good enough.

And for my all-time bonehead moment. Engine rebuild 25 years agon in my 1963 Volvo. Weeks upon weeks of wwork pulling the engine, having it bored to 89 mm (B18 - B20 rebuild). Crankshaft machining, parts ordering. Careful assembly. And reinstalling eveything just right. Started it up, and everything sounded perfect, ran great. I let it run for a few minutes and shut it off. decided to go for a gentle test drive. When I turned the key on I noticed the oil light wasn't on. I looked under the hood and saw that the push-on wire terminal was looking sprung and had fallen off (I think I mashed it with the exhaust manifold during final reasembly). I tried putting it back on, burnt myself on the manifold, got it to sort of hang on, but decided to go for a test drive anyhow (noticing this and not fixing it right makes this a true bonehead series of events, vs. just an unfortunate sequence of events. So I started it up, took it on a gentle drive. Felt great. I didn't drive it hard but it felt stronger than it had as a worn out B18. Until it started to lose power and slow down. And it started squeaking. I pulled over and it immediately died. I opened the hood and the engine was very hot, though it still had coolant. I checked the oil and it was EMPTY. :^O Where had it gone? I looked underneath and the drain plug was missing. And of course the oil sensor wire had come loose again, before the oil plug fell out. Upon some reflection I realized that at the very beginning of the process I had removed the drain plug to drain the old oil, and had just finger retightened it. And during reassembly it never occurred to me to tighten it again, with it sitting in it's place looking all proper. End result after a tow home and a teardown was a burnt bearing on rod #2. Both the crank and the rod were blue, and I had to get another set of rods and a new crank for the re-rebuild. :(
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I'm JohnMc, and I approved this message.