Sounds like a combination of things:
1. Contaminates in the bucket prevented good mixing.
2. It was simply too wet.
3. You tried to add water to mixed thinset after it had set for a while.
Are you adequately cleaning out your buckets when you're done with a batch? Are you using clean water?
The more you work with this stuff the faster you realize you get a "feel" for how much water to use simply by looking at how much dry thinset is in the bucket. Roughly 5-6 quarts for a 50 lb bag is required, so you guess based on how much you need to use. If it's too thin after mixing it add some dry thinset to it until it mixes to the right consistency, but if you wait for 10 minutes or so after mixing and add water you may not get it to mix correctly.
It should be firm enough to stay on your margin trowel, but comb out with a notch trowel without tearing. If I have ANY doubts about the batch I just mixed I throw it away. It's a lot cheaper to buy more thinset than it is to spend hours tearing up and resetting tile I already set.
The tiles you set with that questionable thinset may not stay adhered. The thinset may be brittle. You'll have to test them by tapping on them to see if they stay adhered, or time will tell...
* Corey Graham - Molly is now 6yrs old and started school this year! Morgan (my 3yr old son) thinks papa setting tile looks funny