I think that feeding the wrong things are one thing that may happen if someone does absolutely no research. Honestly, do hatchling corns look like they can eat a pinky? Not really, but they can.
Buying without doing any research, or doing very little, is also a bad idea. I did buttloads of research before I got my corn, and I got many compliments for that.
Perhaps a less important thing I think beginners overlook is the morphs. Either they don't know there are so many morphs, or they do and only want a less common morph (even though I think the normal/wild/classic/Carolina morph is one of the best).
Improperly co-habbing is another. Beginners probably shouldn't do it at all, but I've read a few counts of co-habbing corns, and if the animals are healthy, similar size, feeding, and have plenty of hides, I see no reason corns should be kept individually, honest. If they are adults, then yeah, there's the problem of unwanted breeding, but besides that, what is the REAL reason corns shouldn't be co-habbed if they are good with the terms I listed above? They are solitary by nature? What's natural about keeping animals in boxes? (Little bit of a rant here, don't bother poking me about it.)
Feeding live, and feeding in the viv is probably another thing that probably can be avoided for the better.