I don't think this is every single item I regret buying, but these are what I could pick out from my current collection. I'll do a part 2 if I ever find more!
e.l.f. Luscious Liquid Lipstick "Maple Sugar": I ordered this color on accident. The product itself is okay. I like the mint smell, but it smells a little herbal at the same time. The minty-ness agitates my lips.
NYC 16HR Smooch-Proof Lipstain: I bought this because cosmeticcandiesfyi mentioned she REALLY loved these lipstains, and that they're very cheap. It's not a bad product, I just chose a bad shade and stored it incorrectly (leave the tip facing down to let the stain drip to it. Otherwise, it'll just dry out). Nude's never a good choice for lip stains, I guess.
Urban Decay 24/7 Waterproof Liquid Liner: I HATE THIS! So not worth the $19 I paid for it. Temptalia gave this product an A, so I thought it'd be worth a try. It reminds me of a L'oreal liquid liner I bought a while ago. It's not waterproof at all. I like the color selection in this line. When it comes to black liners, I expect the best. I would repurchase but only in a unique shade.
Neutrogena Naturals Purifying Facial Cleanser: Another product I hate. I used this twice and it was so painful. Neutrogena products have not worked for me whatsoever. They all have ripped my skin really badly, and this just followed the tradition. It's weird that it claims to be a natural product, but has a really sickly perfume scent to it. The ingredients aren't very natural either.
Inglot AMC Mousse Foundation: I went to an Inglot counter at Macy's in SF and the sales associate told me this is the fullest coverage product that Inglot has... When I tried it in store, it looked opaque, however my stupid self forgot I was already wearing foundation and concealer! D'oh! This isn't opaque whatsoever. It's definitely a light coverage foundation.
e.l.f Mineral Blush: This was my first loose powder type of product. The product itself isn't bad, and the color is really pretty. I just really hate working with loose products. It's unbelievably messy.
Laura Mercier Invisible Loose Setting Powder: This is my sister's favorite powder, and Temptalia gave this an A as well. It definitely mattifies the face, however I have normal to dry skin, and it made my make-up very uncomfortable to wear. I liked that it made my creamy foundations smooth and matte, but what happens to me is I end up producing a lot more oils because my skin was sucked dry of any oil! For some reason my sister's works very nicely. I think I got a bad one.
e.l.f. Brush Shampoo: Eh. It's basically just hand soap. This was a really unnecessary purchase.
e.l.f. Cream Eyeliner: There's always so much excess packaging for e.l.f. studio products in pots. This product dried out in a week.
Wet'n'Wild Trio in "Walking On Eggshells": When these first came out, I was really excited because many bloggers were raving about them. They're soft and almost buttery, with good color payoff. They're not a bad product, I just don't like the colors. The pink and white color are packed with shimmer, and the brown doesn't show up well on my eyes. Supposedly they're MAC dupes. I'd buy a different trio if I found colors I liked.