When there had been two requests to download one message at the same time
the later waits for the former to finish, then it reads the message
from the local cache. The problem is that the later can use the same
stream as the former, but this stream is re-winded at the end, thus
the later attempt doesn't read anything and produces basically empty
message, instead of the proper content.

This could be sometimes observed when viewing message source of
a message which was not downloaded yet.