Added a variable to a configuration, set it's value to "true", "prompt as selection box", set "allow empty" to be "false", add two choices "true" and "false". Kick off a build, you're prompted for the value of the variable with the value defaulting to being empty. Click "OK" and the build will continue, despite "allow empty" set to "false". This seems broken.

This occured when using QB v2.0.12 (version list above doesn't seem to list any recent v2 versions)