Sometimes betas miss things too, and I'm speaking as a beta. There have been times I've missed things I should have caught. The simple fact is, no matter how many times something is read through, or by how many pairs of eyes, some things still get overlooked.

I usually only use betas for some of my longer fics, and not for drabbles, but I usually do several edits on everything I write and still manage to miss silly things. I posted a drabble once and then realised I'd left the 'r' out of Thursday.

I would never have guessed you're not a native English speaker, you're English is better than that of a lot of people who claim English as their first language.