I've used these many times before, always followed the instructions (3 tablets/Liter, shake, wait 30 minutes minimum) and never had (or noticed) any issues. I've always been under the impression, that second to boiling it was the most effective method of purification.
However, A few weeks ago I hiked a few days on the AT using iodine tablets as my primary method of purification. When I got home, I forgot about the water in my camel pack for a couple weeks. When I finally went to clean my camel pack looked like I was raising sea monkey's in there.
Has anyone had the same experience? Could it be the quality of the tablet? I'm well aware that even the best polymers aren't impermeable on an extended timeline, but this was only a few weeks, and whatever it was, there was a lot of it.