As of a few weeks, the awesomebar autocompletes "about:blank" to "about:blank/", which gives me an error page instead of the blank page.
mak says, this is likely P1.
I actually thought this was a problem for all about autofills (thus P1), but it only affects blank, that is... funny. We should check what's up, maybe it's also affecting other cases.
I can reproduce easily if I have a bookmark to about:blank