After many purchases, I have a few items which were for me were purchase mistakes.
Vaude Poncho - I like poncho's but have found out from experience that the Vaude Ponch and others like it, where they have to be pulled over one's head and over the rucksack is a real faff. Getting it right means I have to become a contortionist! If I ever buy another one it will be with buttons, studs or a full zip DOWN THE FRONT of the garment.
Waterproof trousers - Can't stand them, much prefer a rain skirt and gaiters to keep me dry.
Paramo Alta 2 - For me a £250 letdown, heavy, unsuitable for the summer and doesn't roll up small.
Stratosphere Bivvy - I couldn't cope with the covered head area, claustrophobic.
Alpkit Hunker Bivvy - I didn't find it easy to use until I fitted a 4' waterproof zip along one side. It's now great.
Underwear - My M & S day to day ones fell apart when hiking, my £3.99 ones from Decathlon are near perfect.
Waterproof ear covering warm hats - My £25 Lowe Alpine hat was great (I lost it) but my £10 Highlander, almost the same design, is definitely better.
Gloves - mine were always wet inside until I designed my own glove covering tubes. Two waterproof coat sleeves cut off at the elbow, now my gloves stay dry.
Decathlon Phone/cards Pouch - great pouch, useless neckband. A new 1" wide and soft neckband sewed on makes all the difference. I can now wear it and not realise its there until the items are needed.
All items just personal to me.