When writing your first draft, are you really supposed to never look back and keep writing

I'm starting to write up my first draft, and in the middle of writing a few hundred words, sometimes I instinctively realized I wrote something very subpar that, if I spend minute or two thinking about, I could probably make better. However, I hear often that you're supposed to keep writing regardless when it comes to first drafts.

Have you ever written a paragraph in your first draft only to immediately realize you wrote something bad and go back to fix it? Or do you just tough it out and keep writing until your brain stops?