generally speaking....
1 speakers whose binding posts are separated too far apart. von S 4JR were 20 inches or so really stretching the bi wires i had at the time. or forcing the use of jumpers or shotgun wiring.
2 preamps whose on/off switches used toggle switches similar to knife switches that were thin and extend from the face plate inordinately far. they were prone to breaking off in shipping. and, did.
3 getting upscale ubber sensitive audio gear and realizing you have a ground loop issue from your CATV rig and having to chase that down.
4 sub woofers whose controls are all on their rear panel and have no remote controls.
5 thinking a new ??? will fix the issue you have but then finding out it was merrely a synergy issue with miss matched components.
6 being told a prospective amp is fresh out of the box and that it will ease up after break in, but finding out that is the way it sounds once broken in after all!
7 eg., above but with speakers.
8 eg., same as above with anything else.
9 hearing that soft swoosh as the preamp volume is controlled up or down with the remote. Although it stops once you are at the what ever volume, but it does tend to annoy at times.
10 owning mannually biased mono amps…. and suffering from bias checking anxiety.
11 Buying a new pre/pro or receiver whose operation manual is more than 150 pages. actually if its more than 30 or 40 will be enough to lite the irritation fires.
lastly, components whose IEC inlets are all over the place, right hand side, left hand side, and or in the middle making for dressing power cords a real mess.