Yes, a bit surprising, but yeah, even Google...! I'm talking about the classical GMail interface, which is still regular (the new one has not yet been put as a standard...)
While trying to reply to an email, or even write a new one, here is the warning that is being displayed:
Meaning: "impossible to load the rich text editor".
It is then impossible to write at least within the data part of the email. If you were going to write a new one, you won't be able to write the recipients'addresses nor the subject.
Update 1, 01/15/12:
Bug fixed in version 9...