For as long as I can remember, whenever I’m typing out a letter, or a long forum post, when reading it back, I often notice I’ve missed out one or two complete words. The spelling and punctuation are usually ok (usually!), so I’m obviously paying attention to what I’m writing, so why does this happen? Is there a medical term for it?
Happens to me too. I think the casue is that my brain is faster than my fingers and so when the brain gets too mmany words ahead, the fingers just drop a couple to catch up.
“Proofread carefully to see if you any words out.” -attributed to William Safire 
I’m amazed that no one has posted a sentence leaving out an obvious word yet.
now people will scrutinize this post to try to find the non-existant missing word.
Simulpost…yeah, that’s it.
Yeah I do it too. When I’m writing, I continually repeat the sentence over and over in my head - so maybe it’s that some writers think in terms of sentence units whereas others build word by word… I dunno.