I remember reading an author's comment that in their stories, they always have to check that children age. I suppose we all have our own weaknesses or tendencies.
I am creating characters at the moment, and having lots of fun doing so. I have to check that I don't create families that are too similar - two parents, two point four children - but on the whole it's going well. I know I tend towards stereotyping, and I have to consciously stop and check for that, and sometimes insert interesting quirks, such as a phobia, to make a character more memorable.
During a rare time alone today (in the car, trying to source cereal and milk at 9pm on a bank holiday Monday, while S was at home with the littlies) I thought about two things I often forget: facial hair and pets. None of my characters seem to have either. Beards are funny old beasties, though, aren't they? The consensus is that they're not really sexy, though you might see a photo of a bearded man and think otherwise; but the word 'beard' is more likely to conjure a picture of Santa or Great-Uncle Victor than the gorgeous mystery man you saw squeezing tomatoes at Sainsburys. Of course, I could just give Great-Uncle Maurice a beard in my novel, but then I'd be falling into a stereotype again.
As for pets, I suppose the lack of them in my writing reflects the lack of them in our home (we have children instead of pets at the moment. In a few years, when the children require less hands-on input, we may have time to remember to feed, walk and toilet a four-legged creature, but for now I can't even keep a cactus alive as well as the children).
Anyway, I shall be going through my characters at some point and seeing if there is anyone deserving of a beard or a three-legged hamster. Perhaps the one might even be mistaken for the other...but no, it's not that sort of story.
What do you forget to include in your character's backgrounds/appearance when you are inventing?