One of the more dispiriting realisations I've had today (which has otherwise been quite a good one) is that almost every time I look over anything I've writing in the past, I discover some typo or grammar error I've made, and I'm a stickler for avoiding such things. I know better, but I've had to accept that I'm blind to these things when proof reading. Sigh.
@lightweight I try to be careful about this as well, yet always find obvious mistakes later too. During review our minds show us what we meant to write instead of what we actually wrote.
As suggested, I find changing how the text looks helps. This could mean changing colours, typeface or zoom level. If the source is Markdown or similar I find reviewing the rendered output helps (and then reviewing the source again after tends to find more problems).
@lightweight An experienced writer told me that reviewing from the bottom up helps them find otherwise masked problems. Read the last sentence, then the second last and so on. I haven't tried this seriously but probably should.