It would be hard for me to answer. I've never bought a piece of gear that I thought was a mistake at the time. Plus, many things I've bought were things that I thought were pretty good at the time; but looking back in the rear view mirror, today they look pretty sad. But that's where technology was at the time, which is quite different from what we have today.
CE2s are a good example. They were my next step after cartomizers. I thought they were fantastic, at the time. But looking back; yeah, they were pretty bad.
After that, I became a big fan of genisis RBAs for a time. The first one I bought was a Chinese knock off that we used to call a 'DIDalike'. It was a knock off of an authentic genny called a 'DID'. It was a pain to build on, because you had to have fingers like Kermit the Frog. But once it was set up, it worked; and it was the device I learned to set up mesh wicks on. It led me to buying several much better gennys later.