-1: Fucking up the stats.
No, seriously, you'd be surprised how many people screw this up. They either don't realise that the basic gamer fic is a numbers game, and then scratch their ass in lieu of a brain case while they wonder why people get pissed when their protagonists hard won power progression up and disappears without explanation, or they do something completely asinine like introducing charisma as a stat, or having dex replaced with agil, or even worse, listing the two stats together on the same damn sheet. Now, it's gotta be said, this is stupid. Like, mouth breathing, red-neck for six generations, your family tree resembles a stick, stupid.
To put it plainly, this happens for one of three reasons.
The 1st of these, is that it's only somewhat a Gamer fic, and the system is based on something else, like Dark Souls stats or something, which is fine but remember to put an authors note or something that hints at what we're in for.
The 2nd of these, the one that is somewhat acceptable, is that the character itself is deficient and as such the representation of their gamer stats reflect this, and either later change or have by that point become mentally ingrained to the point they are no longer questioned.
The 3rd reason however, is dumb, and it comes about when the author is to stupid to understand the difference between base and derived statistics. We have strength and dexterity, two attributes that are examples of how the now-conceptual existence that is the Gamer interacts with the world. The Gamer is proven to be a conceptual existence, and their stats to be conceptual attributes, because of their ability to say, punch a mountain in half without spontaneously becoming a mountain of muscle, ala the the hulk. Agility on the other hand, is a measure of how one applies ones Strength and Dexterity in unison in order to navigate the world. It is a derived statistic, the same with Charisma, which is derived from applying Intelligence and Wisdom toward social situations, multiplied by relevant skills that affect such things, said skills often being instinctive, like how people tend to case a room on entering in order to read the emotional atmosphere and not make an ass of themselves. So Charisma = (INT+WIS) x Relevant Passives. Easy, in fact it's impossible to fail at, unless your one of those fucking morons.