Upon fiddling with a RealURL config, I noticed that I had unknowingly turned off RealURL caching, as in this old post http://blog.marit.ag/2007/07/24/realurl-pathcache-ausschalten/
As of 2015, are there still reasons to disable RealURL Caches?
Urs, I have no idea why blog's author gave that statement but it's generally wrong. Have no idea what kind of problems they encountered (maybe if they described it, it had some sense), anyway best solution is proper RU configuration instead of disabling the cache.
Keep in mind that general rule in TYPO3 is - cache, as much as possible, as long as possible.
If you'll meet some problems, then you can just clear the RU cache (currently probably by truncating RU's tables in DB, as dedicated ext have some problems with TYPO3 ver. 6.2) and try to fix the configs.
I just checked RU configs in several installations for my own curiosity, to check if is there is somewhere disabled RU cache, result: 0.