This has been happening a lot lately. If I'm typing something on a Plus! window (whether it be the Messenger Lock password dialog, script editor or on a script window), sometimes, a key will get stuck, in the sense that it will act as if I'm holding it down. All windows in the Messenger process freeze whilst this happens. In some cases, it'll stop after a while, other cases I have to end the process.
Latest versions of Plus! and WLM, running Windows 7 Home Premium x64.