The mistake is buying a leather bag by the photo on a hanger hook, then getting it home and finding the strap drop puts it at a weird spot on your hip, or the opening is so narrow you're fishing for your card with two fingers in a bus queue. Leather doesn't forgive a bad shape the way canvas does. It holds its structure, which is the whole point of it, but that means whatever's wrong with the fit on day one is still wrong on day two hundred.
There are places a leather bag simply isn't the right call. If you need something that squashes flat for a cabin bag or folds into a drawer, leather fights you, it has its own opinion about how it wants to sit. It's not the one for festivals, beach days, or anything that might get properly rained on without a chance to dry out slowly, water marks leather in a way it doesn't mark most synthetics. And if you're after something genuinely light for a bad shoulder or back, a lot of leather bags, especially structured ones, carry more weight empty than you'd expect before you've even put your life in them. For fit, pay attention to the strap drop against your actual height rather than a model shot, a crossbody that sits mid hip on someone five foot eight can sit at your thigh if you're five foot two, and that changes how it swings when you walk. Check the opening width too, a zip or clasp that looks secure in a photo can still mean posting your phone in at an angle every single time, which gets old fast. Shoulder bags should sit without the strap biting into your shoulder when your arms are full of shopping, if it's already digging in empty, it will dig in loaded.
Good leather has a bit of give under your thumb and a smell that's actually leather rather than a chemical approximation of it, cheap leather tends to feel papery at the edges and crease in a way that doesn't bounce back. Check the stitching along the base and handles, it should be even and tight, not spaced out like someone was rushing to finish the row. Hardware matters more than people expect, zips that catch, clasps that feel light and tinny, or a lining that's already fraying at the seams on a new bag are all signs of a disappointing one. A good leather bag shouldn't need babying every time you set it down.
Where it clashes is usually with itself, a heavily structured leather bag alongside a stiff coat or structured blazer can start to look like armour rather than an outfit, so I'd pair it with something softer in the rest of the look.
If I were starting from nothing, I'd pick a mid brown over black, it goes with more than people assume and shows less of the general scuffing that comes with actually using a bag.