If you move a forum topic from a standard forum to a one-topic-forum the topic is moved but not available. You have to change the forum typ setting to standard to see your topic again.

There should be a warning that the topic is moved to a forum where it can't be seen. An alternative could be that it is not possible to move in forum with this settings.

Saw this behaviour in a 1.9.1 system. But I think it will be so in all versions.