This is probably the most common mistake I have seen made in any form of writing (and despite how minor it appears, it severely annoys me because I am OCD about commas, and everywhere I look, I seem to encounter people violating this rule):
<subject> <predicate1>, and <predicate2>...
Example (from on-site):
"Although its previous tier placements may indicate otherwise, Alakazam has always been a Pokemon with purpose, and
has now returned to its old stomping grounds in OU."
The comma is completely unnecessary because the conjunction "and" does not combine two sentences; it combines two verb phrases that are not complete sentences. Commas should not be used to divide a compound predicate
except in cases where they are necessary to provide clarity. This is also intuitive; when saying that sentence out loud, there would naturally be no pause.
I therefore want to suggest that this rule be added to the OP.