I remember one time at the seated rowing cable machine, the intensity was hitting so good after weighted pull-ups. But for some reason my vision was going black after completing a set. It felt like I was going to "faint", while still being fully conscious. I couldn't see for like 4-5 seconds. I sat in place, just focused on deep breathes and calming my nervous system down, and in about 30 seconds everything went back to normal like nothing ever happened.