Last night I woke up and felt a tingling in my groin. I focused on the feeling and started feeling involuntary contractions and the intensity of the sensations started ramping up.
My cock suddenly got hard and I could feel a small amount of ejaculate coming out. It was only a few drops but was not pre-cum.
Source: https://www.reddit.com/r/aneros/comments/hr2ec0/first_hfo_maybe_last_night_aless/
It also happened to me one night, I went to bed with my MGX and at night I was woken up by a strong boner in an ejaculant. This is cool.