There is a transatlantic difference here—at least with respect to traditional grammar.
Very often US English uses simple present where UK English uses present perfect. For example, in a comment, Dan Bron says:
Both "I just did my homework" and "I just finished watching the movie"
are grammatically correct and perfectly normal; yesterday someone said
"Do you want lunch?" and I said "No, I just finished my breakfast".
I wouldn't use any of those expressions, I would say:
"I've just done my homework"
"I've just finished watching the movie"
and
"Do you want lunch?" "No, I've just finished my breakfast"
Of course Britons are influenced by US English especially in the popular media so we understand it perfectly well but this form is usually a clear indicator that the writer is from North America. (My opinion)
P.S.
I can certainly imagine some Brits writing, WTF did I just see? as a comment after watching a Youtube video. It's shorter and, dare I say 'cooler', than WTF have I just seen? It would however be written knowingly and in imitation of a US-type comment.
A more British version would be, WTF have I just been watching?