A very informative thread, thanks for starting it, OP.
I almost always regret an impulse purchase. I'm an impulsive gal, therefore many regrets. Here they are in no particular order (no offense meant to anyone, please):
1)
Gucci pochette (from a LOONNNGGG time ago) zipper broke on first use.
2) Gucci pelham hobo - fabric was not a good quality, the corners seemed to fray the instant it came against any hard surface.
3) Chanel Cambon tote - looked plastic-y.
4) Chanel GST - not functional with the middle pocket.
5) Chloe Heloise Large - ugh... I dunno... didn't do it for me.
6) Chloe Paraty - same as above
(However, I love the Paddington even though it's a little heavy)
7)
Fendi B bag - cumbersome, stiff.
8)
Fendi Spy bag - I can't explain why I just got sick of looking at it after a couple of hours.
9) Louis Vuitton Palermo PM - really hard to wear on shoulder with the wider bottom against the hips.
10) Balenciaga Day - not exactly the most disappointing bag since I love the leather but this was SGH and it was heavy. Also, hard to find anything in the bag. I would have used it as a book bag but it was too heavy with books in it.
11) My mom's black Hermes Birkin. I was too young at the time and it made me feel too "mature". Also, it was rather structured.
12) Dior saddle bag - Too trendy, too small, oddly shaped, totally unfunctional for my needs. One of the worst bag purchases I've ever made.
That's all I can think of at the moment, though I'm sure there are many more.