This has happened with me as well.

Occasionally i will remove the pin by pulling on the barrel and plunger (depending on location of inject) at the same time creating just enough draw to allow a bit of blood to enter from capillaries on the way out. This is MY theory only. But it explains (to me at least) the blood in the barrel after the pin is removed. Again, this is rare but it does happen.

However, it's nothing to be alarmed about as already stated above.