In Debian, if there exists a bug in a version of a package in SID (let’s take as an example GRUB2 version 1.97+20091125-1), and it has a severity marked as Critical, then it won’t touch testing until such a bug is fixed… It is as simple as that.
What happen when?:
a) You are not sure which Debian release you are using.
b) You are not sure how Debian handles bugs reported with severity: critical.