OK, so not a bug per se, but it does 'bug' me...
On the Data Explorer page, the link to log in is labelled login rather than log in. The latter is on all the other SE sites I clicked through to, so why not the Data Explorer?
After all, LoginIsNotAVerb.com!
EDIT 1: Also noticed that when logged in to any SE site, the link to log out is labelled logout, but should be log out. Hover over your username to see this link in the summary that pops up.
EDIT 2: Just to clarify, this question is not about whether "login" and "logout" are verbs - I think that's pretty well-defined already. The question is can we make sure the correct forms of log in and log out are used consistently across all the Stack Exchange sites?