Oh, where to begin... biggest mistake is probably buying the Cleveland UHX irons. Saw some images online and fell in love with them. Decided to order a set without seeing them in person. When they arrived I legitimately thought they had sent me a set of hybrid irons by mistake. Don't get me wrong they are not bad irons but the look was a big no for me. Plus they are meant to help you get the ball up which is not an issue I have. I always joke that I can hit my putter high.
Another would probably be not just sticking with a set and figuring it out. In my early 20's I kept golf shops in business with the amount of trade-ins and pre-owned equipment I bought. Now that I have taken the time to work on my game I can see it was the swing, not the set that was the problem.
Then there was the square driver era. Buying the Callaway FT-iQ, Wilson Staff Spine and Nike SasQuatch were big big mistakes. No idea why these were a thing but I am glad they are not anymore.
And lastly was thinking anyone but Adams knew how to make a hybrid in the beginning. Ok, that one is a joke but my dad still has an early Adams hybrid in the bag and every time we play I will hit it at least once. That 15-year-old Adams Idea a7OS technology still holds up against a Callaway Apex UW that is only a few years old.