Ubuntu 10.04 hit by major bug on release day
As it turns out, after the final release ISOs were already created (or "spun" in Ubuntu geek-speak), a critical bug came up; and no, I don't mean something like "OMG, we put the window buttons on the wrong side!". I mean something like "once you install Ubuntu, you will not be able to boot your other operating system." Oops. It doesn't kill the other OS, and it's fixable when it happens (it's a problem with the GRUB2 bootloader configuration), but it's certainly not something you want to have happen to people just trying out your OS (or even upgrading from previous versions).
Granted -- it doesn't happen with each and every installation, but it can happen. The Ubuntu team considered a quick-fix, but eventually went for the more responsible (and safer, in terms of PR) option of re-spinning the ISOs. However, re-spinning all ISOs would push the release into May, which would be a PR flop in itself. So they're just re-spinning some of the most popular ones, like the x86 and x64 desktop releases, as well as the Netbook remix. Exciting stuff!