Strange "1;2600;0c" sequence in startup (message-mode?)

[Jesse Sheidlower]

I have recently been seeing a strange sequence of characters when I
start Emacs up to send a message, either when sending a new message, or
replying to an existing one. I believe that this started when I began to
use message-mode. Typically it will be "1;2600;0c", inserted at the
start of a message, but I have noticed other numbers close to "2600" in
the middle section.

This only happens when I have started typing before Emacs has fully
launched (usually by hitting return once or twice); if I wait for it to
load, I do not get any such sequence. But it does happen very regularly
when

It's been hard to Google for this. Does this ring a bell for anyone? I
have some hooks for message-mode, but nothing that looks like it could
cause this, although I don't know what the sequence represents.

Thanks.

Jesse Sheidlower