RC builds are generally considered stable, but it looks like one bug made it into CyanogenMod 10.1 RC3. There is new code in the ROM that does calculations for various parts of the software, but apparently it was only tested on one chipset. If you aren’t using that chipset, it’ll cause all sorts of problems. They say that you may see animation speeds go crazy, mobile data readings show huge amounts, and more. A big worry for some is the fact that they checked data usage and saw massive amounts.
However, rest assured that all these issues are only cosmetic. Whatever you saw was just bad calculation due to the new code, so you didn’t actually use 20GB. They say that they fixed the bug in RC4, so get downloading! And our teachers said math is good for us… right.