I cut the ends off the (excessively long) bolts that attach the bike rack to the roof bars. I thought that the time it took to do so would be repaid in time saved when fitting/removing the racks. You know: by not having to screw the nut down over the unnecessary 10-15mm of bolt and back. Also, it was a satisfying thing to do.
Several years and a car or so later, the roof bars have been replaced by 'aero' bars. I discovered on the morning of our camping trip that the new bars are a deal deeper than the old ones, and the sawn-off brackets are now too short.
I spent about an hour looking for suitable replacements in my various "that'll come in handy one day" stashes before accepting that I didn't have adequate replacements. One trip to screwfix (and another 40 mins or so) later I have some bolts that will just about do, but not for all of the fitting points. So I finish the job off (suboptimally, but sufficiently) with toe clip (and other kinds of) straps - adding another 20 mins or so.
So:
Time saved over the years by 'trimming' the original bolts = (at very best) 60 mins.
Time spent trimming the original bolts = 12 mins
Time lost attempting to replace them = 120 mins.
Result? Misery.
And it's not over yet. I wasn't happy doing more than 50 mph with the frankenfixing, so I'll still have to find a better bracket solution.