I thought it would be fun, instructive and a good waste of time to have a thread about the stupid things people have done to Hewlands. Here's my list, not all of which I did, but all of which I have suffered from...
Forget to torque the side fill plug and spew Redline ghastly blue stuff all over the inside of a Merlyn Mk 6A sports racer rear body half. Took hours to clean...to be honest, there was still some on it when I sold the car two years later, if you looked close. Last time I used Redline ghastly blue stuff...
Have a new pinion installed, get the fifth gear endfloat off by about .100" and take hours to figure out why you can't engage 5th gear.
Have the pinion nut come loose and find out exactly what happens when you run for a while with the pinion wobbling all around (just prior to getting the fifth gear end-float wrong above). (what happens is you get a spare MK4 pinion useful to help set the shift forks on the next Mk4 gearbox you work on, which is approximately never)
Leave the circlip off of an output shaft and have the shaft walk out during a session.
On a one-off custom installation make up a clutch bobbin spacer out of aluminium, and have it run dry and gall itself to the input shaft. Take gearbox out, take diff out, get the input shaft out, beat the spacer off with a big hammer, clearance the spacer ID, file/scrape/chisel the spacer mung that's welded itself to the input shaft off, lube the crap out of it with anti-seize and put it all back together and make the next session. (lesson learned is that 6061-T6 is the wrong material to make a clutch bobbin spacer from - it will last just long enough to lull you into complacency, then fail,,,)