The fact that the vast majority of affordable bikinis are still sold by dress size, and as a set .... as opposed to by bra size and pant size as separates (unless you go to one of the premium and very expensive brands).
And as a soon to be mother of the groom looking for an appropriate but not frumpy 'occasion' dress suitable for a very hot location, this idea that summer dresses = backless (and so low that even a so-called backless bra wouldn't do the job) and/or cut out at the side has frustrated me beyond words. Beautiful from the front .... completely impractical unless you're flat chested and can dispense with a bra. Honestly probably a good 2/3 of such dresses seem to be designed that way currently.
And related to the above, and online shopping, retailers who don't provide photos of said garment from all angles so you know what you're getting - or not, as the case may be. Meaning you potentially waste your time - and theirs - by taking a chance on ordering something which may not be right.
Similarly, retailers whose online photos are way off kilter colour wise compared to the item in real life. Appreciate this isn't an exact science but have often found stark differences ... maybe due to light conditions if photos taken in a sunny location for example.
And bloody supermarkets who sell clothes getting rid of their changing rooms. I don't care about your refund policy ... I care about having to spend extra money on a range of sizes and colours to find something that suits, and the time and hassle involved in returning stuff.