Just had one of those days so my list is:
Products that don't have captive screws;
The cross head screws in Chinese imports that do not fit any known screwdriver so you have striped the head getting them out before trying to refit them;
The screws made of grey cheese that strip the head or shear off as soon as you look at them, usually supplied with Chinese imports;
Enclosures without bus bar, neutral lead, lables and any instructions saying what BS standard they are manufactured to but they do have a neutral and earth bar!;
Box sinkers with little Allen bolts that fall out all the time;
Choc box type connectors with a nice ledge inside so that multi core or flex pushes at least one of the strands back, assuming you can get them in in the first place;
Consumer units that can take 25mm tails in the switch but with no room to bend/fit them unless you drill holes to allow them to come straight in;
Flimsily consumer units and RCBs that threaten to break if you try to use 25mm tails;
Accessories that have a shroud in front of the connector too small to allow more than one insulated wire to be pushed in so you have to have bare copper at the entrance to the shroud;
20mm plastic conduit fittings that cannot cover a 20mm hole;
Consumer units with levels that are not level;
MCB, RCDs etc with a very short cable clamp so the cable can fall out very easily, especially 25mm tails!
Light fittings that you have to hold up, they are usually heavy, with one hand while trying to get the connections into the ****** choc box connectors with the other two hands;
Line bus bars without any shrouding;
Plastic accessories with knockouts that are just a little larger than 20mm so you have to use silicon (ugh) to seal them.
Have I missed any accessory/fitting off my list as I seem to have problems with most modern fittings even the more expensive ones. I have not named names as I would be here all night and end up being sued by nearly every supplier in the UK.
I have not had any problems with Wagos, Wago boxes and Ashley MF free junction boxes. There are probably lots of other things that I would like, I just never seem to find them.
Since seeing the posting about fake CE marking I have noticed fake CE marks on a number of products that claim to have BS certification!