If you view your completed tasks (http://www.rememberthemilk.com/help/answers/tasks/completedtasks.rtm), you can select the task and then mark it as uncompleted (through the dropdown menu, I think).

If you completed it on the website, you can make use of the Undo feature (which lasts as long as your session, e.g. when you log out or close your browser).

But yes, you can find it in your completed tasks also (you could just search for it too), since RTM doesn't delete your tasks unless you ask to.

Thanks for the help guys. I found the task again.

Regards
Christian