A previous question discusses whether men should avoid showering after using the Mikveh. I have heard specifically that the Ari Z"l frowned upon this. Can anyone please quote the source, if one does exist?
In tractate Gittin, I'll try to find exactly where, there is mention of a rabbinic decree that actually caused drawn water to bring on ritual impurity. Rashi explains the reason.
Mikvehs used to be in caves, where the water could get, for lack of a better word, yucky. The rabbis saw that people would regularly bathe after going and wanted to make it clear that the post-immersion bathing played no part in the purification process.
How that translates to halacha nowadays, I have no idea, but this gemarah came to mind when I saw this question.
Edit: Here's the source: http://www.dafyomi.org/index.php?masechta=gitin&daf=16a
10 lines down, and in Rashi, who brings down shabbos 14a:
14 lines down.