I suppose it had to happen at least once. For the first time in my long history of donating blood and platelets, a donation failed. The return needle just wasn't getting into the vein for whatever reason. Not a good thing, because you want the blood going back into the vein and not into the arm, which is painful.
After a few attempts to reposition the needle and to find another vein to use (apparently all my other veins are pretty deep and the other easily accessible ones are too small), they decided more messing around would cause more damage than it was worth.
Not sure what the problem was, but I suspect I just didn't hydrate enough after the run I went on just before going to donate.
Will give it another go next month.
The bruise on my left arm is going to attract some attention I think.