WIT always creates a 0 sized lock file when you edit a document : document.wit.lock if your document is named document.wit. The lock file is located in your document folder.

This is useful to prevent concurrent access by other users, and the lock is removed when the document is closed. A locked document can't be saved to avoid any change on the file.

Sometimes, if the WIT session is killed in any way, the lock remains : you just have to remove the null size document.wit.lock file.