Different people have different outlooks, and mistakes for one person may not apply to all. That said, one of my main mistakes was buying serrated knives. Back then, I mostly wanted knives for self defense (not living in a state where I could get a CCW), and I thought Cold Steel serrated knives were the hot setup. That was a mistake. I'm still fond of my serrated Vaquero Grande, but I misjudged the limitations of serrations. Also, like many others, I made the mistake of buying half-serrated blades. Yeeech! Now that said, I did buy a couple of Spyderco Enduras that had half-serrations, but I like them.
The biggest problem with serrations is that you can't cut wood or paper products very well.
Another mistake was thinking that I needed expensive knives. I found that Cold Steel made some very nice knives at reasonable prices, and I was particularly fond of the Voyager series. CS also is one of the only manufacturers that make large folders. I tend to like 4-6-inch blade folders and have little use for 3.5-inch and smaller bladed knives.
Another pit to avoid falling into is the gorgeous knife that you can't take your eyes off of...but when you buy it, you find that it's far too heavy to carry comfortably. I've got a box full of knives that I dearly love, but I never carry them because they're uncomfortably heavy. Size isn't much of an issue with me, but any metal is gonna be heavy (even if it's aluminum), and some manufacturers hide the metal as inserts to FRN (fiberglass reinforced nylon) and G10 knife handles. Actually, FRN and G10 are plenty strong without inserts. A lot of bikers carry Cold Steel knives with 5- or 6-inch blades, and they drop them going 70 mph or have them run over by trucks; and the things are virtually indestructible. Spyderco's newest Endura 4 doesn't have metal inserts, while the earlier ones did, and there are just no strength issues. King Kong couldn't break them. So get knives you'll be happy carrying.
Heat treat is another thing many knife buyers don't always take into consideration, and steels advertised as "surgical stainless" means they're made of cheap steel that won't keep an edge. And steels advertised as 440 are always 440A, which is the cheapest of the 440s. Spyderco and Cold Steel have outstanding heat treat, but other brands that advertise acceptable steels produce blades that come out of the box dull and can't be sharpened. I bought a Boker Magnum once with "440 steel" and the thing is kicking around in a drawer because after spending about 45 minutes with it and my Sharpmaker, it was as dull as when I started. Every now and again I'll get it out and try to sharpen it, but it's a lost cause. CS made a few knives made with 440A, but they were outstanding. Viciously sharp, I have a couple I still use, so heat treat is everything.