I had a 1977 CJ5 with a 304, T150 3 speed and 2 speed transfer case. I was going to swap in a T18 4 speed transmission. First time I did it, I installed transmission and then bolted on transfer case. Later, went to replace clutch, and I thought I'd leave the transmission and transfer case together as a unit, to save time.
In the absence of a transmission jack, I rolled under on a creeper, unbolted the transmission, and lowered it down to my chest, manually (just how I had done it several times before). The critical difference from previous times was that in the past, I had separated transmission from transfer case, thus dramatically reducing the weight. This time, I ended up pinned under the combined transmission and transfer case.
The jeep was jacked up just high enough that I couldn't roll out with the transmission on my chest, the transmission was caught on the jeep's body, and caught on my shirt. So I couldn't roll, I couldn't get the transmission off of me, and I could barely move...or breathe. Ended up resolving it with a lot of swearing, sweating, and some creative sliding. I separated the two and reinstalled them separately.
The other garage related goof I've had was installing a lock-right locking differential in the rear axle of my first truck, an F-150. Picture a young bachelor doing some truck-improvement. Working in the shop, cell phone on the workbench about 6' away from me, when I found out that the hole for the cross shaft in the differential was just large enough for my finger to fit in, but not quite large enough for my finger to come out (was trying to align diff gears). It was late at night, I was in the shop by myself, couldn't reach the phone or any other way to contact anyone, no one knew I was in the shop working, and no one had any reason to be in the shop (or really to check on me). I was picturing being found a week later, still stuck in the differential, and all of my efforts to remove my finger only seemed to make things worse.
I was contemplating whether I could reach the acetylene torch with my foot and cut the whole pumpkin/ center section out of the axle, when my finger slid out.
That changed my shop practices a bit, regarding telling others where I was, where I would be, when to check on me, etc.