Trained with a guy once who just straight out wasted everyones time. He wasn't very good at anything we did, but this wasn't the problem. The problem was he was lazy, he just didn't try at all.

He would give a half hearted attempt at a technique and give up halfway through it because he didn't think it was going to work perfectly. He was so busy talking and making excuses, he didn't listen or learn at all and generally he displayed no thought to anyone elses training when they were paired up with him, just soaked up 90% of the time on him trying things half heartedly and then talked about why he couldn't do them the other 10% - leaving his partner with no chance of their own. I still wonder why he ever bothered at all.