Retrograde ejaculation occurs if semen is ejaculated from the sac at the base of the penis (seminal vesicle). Instead of passing along the urethra in the penis to the outside, it travels in the other direction and enters the bladder.
It is usually a complication of surgery in the area (e.g. surgery on the prostate), due to advanced diabetes or a side effect of some uncommon drugs. The man has the sensation of orgasm during sex but no ejaculation occurs.