Over-carbing, no doubt. Noobs tend to lack the patience to just set the regulator to the proper pressure for the beer temperature and let it sit quietly for a couple of weeks. Instead they'll seize upon some random recommendation for "burst carbing" which almost always leads to carbonation problems.
Next would be using too short (and/or too large inside diameter) beer lines. You'll find nearly every "beer line calculator" miraculously ends up recommending 5 foot lines using 3/16" ID tubing, yet here we find that folks that follow that advise end up starting yet another thread concerning foamy pours. I recommend 1 foot of good quality 3/16" ID beer line per PSI of serving pressure as a preemptive strike against proliferating "Help! My Pours Are All Foam!" threads.