I'm not sure. I'm still relatively new to Lift but my thought-process was this: I'm seeing lots of settings (esp. LiftRules) that live in singletons. For runtime you just set them once in Boot and it's happy days but when it comes to testing, there've been a few cases I've run into where the singleton settings will initialise before a test can change a needed setting. Easy to fix once you're aware of it, but the problem I had was I'm not always aware of it and sometimes it can take a while to pinpoint the problem. I was thinking if we could let the user know when attempting to set a property that it's futile, it would save effort.

By `RunModeProperty` do you mean just adding something like `warnIfRunModeInitialised()` and getting rid of `PropertyWithModificationCondition`?