We were making Gouda in class today and one of the ladies put her rennet tablet into the milk without dissolving it in the water. The milk was 90 degrees. It never formed a curd. We waited 20 minutes and it was still milk. So we added 4 drops of liquid rennet, diluted in water. We waited 15 minutes and it was slightly thicker, but still milk. So we added 4 more drops diluted in water, and about 15 minutes after that it started to formed a curd.
I guess I've never accidentally added the rennet without diluting it in cold water. I was surprised that it didn't work at all. But now I know.