First time growers frequently:
Overwater their medium based plants.
When you pot your plant, judge its weight dry by hefting. Then, water the plant thoroughly, until water runs through the drain holes. Heft it again. When your pot feels as nearly as light as it did dry, its time to water again.
Overfeed their plants.
As Mr Soul says, beginners rarely under do anything. When all else fails, follow the directions on the fertilizer bottle. [Editor’s note: don’t follow GH’s instructions! Use @ 1/4 their recommended strength.]
Overanalyze their grow.
A first grow is like a first born child: you pay attention to every little thing that happens. Further, you worry at the first sign something may be wrong. Pay attention to what happens in your grow, but do not try and find a remedy for every yellow leaf. Frequently, the remedy of flushing your medium causes more problems than it solves. Be responsive when things go wrong, but be conservative in your remedies.
Overspend on grow supplies.
New growers frequently commit themselves to unrealistic and expensive first grows. It is much more efficient to learn to grow and then invest in high end equipment rather than the other way around. Most experienced growers don’t have room for all of the grow paraphernalia they aren’t using.
Talk about their grow.
Don’t tell anybody that does not have to know. How can you expect anybody else to keep a secret that you cannot.