I think all things watering related has to be in top 3 first time grower errors. This includes using nutes too early and/or too much. Starting off at 1/4 strenght no sooner than 2 weeks after germination is a rule I usually stick by and I use pro-mix which is nuteless. I never used it, but with a beefed up soil like fox farm ocean forest you can probably go even longer I would presume.
Being prepared is another big one. Can't count how many times I've seen frantic help threads where folks order seeds, wait the week or two for them to get there, germ them, then when the seed is breaking soil, freak out because they don't know what kind of lights to buy. Seems like common sense, but don't even think about putting those seeds in a wet papertowel or moist soil before you have some sort of veg light purchased and in place. It's also a good idea to run the lights for a few hours ahead of time just so you know how it's going to affect the temps in your room. It will give to time to make any adjustment while the seeds are still in the pack, rather than scrambling to fix it as the seeds are coming up. Not just with MH/HPS light, but with CFL's too. It's a common misconception that CFL's don't create much heat. Trust me, they do. Not as much as HID lighting of course, but they can heat up a small cab with little or no ventilation quickly.
This one is more of an individual judgement call, but I'd stay away from bagseed. Don't get me wrong, we've all seen excellent grows from bagseed, but in my opinion it's a gamble not worth taking. Especially if you have the money to spend a couple hundred on a hps, top shelf soil, expensive nutrients, etc. Go the extra mile and pick up some decent genetics. They don't have to cost a fortune either. At least then you have an idea what you are growing. For example, a random bagseed from some decent bud might be all or mostly sativa, which probably won't go over well if you are growing in a small closet.