g1smd - 8:52 am on Feb 13, 2013 (gmt 0)
It's not a quirk. It's designed to work that way.
Proper "anchoring" in the RegEx pattern is key to making the rule work the way you want it to.
Depending on what "that" is, there is a possibility of an endless redirect loop appearing for some of your redirects.
You may need to look at THE_REQUEST to make sure you redirect only direct client requests and not redirect previously internally rewritten requests.