A denial of service flaw was reported in OpenTTD 0.3.5 through to 1.1.4 [1],[2].  If a remote attacker were to join a server and start downloading a map file very slowly, it would prevent other users from joining the server until the download was complete.  As well, the game would be paused until the download completed, preventing currently-active users from continuing the game.

This has been corrected in upstream 1.1.5 (via svn r23764 [3]).

[1] http://security.openttd.org/en/CVE-2012-0049
[2] http://bugs.openttd.org/task/4955
[3] http://vcs.openttd.org/svn/changeset/23764