Since before 1.0, we've used "vars.active" to flag whether fans are running or not. Very recently, something has caused this to conflict with the built-in boolean "active", indicating whether an object is being processed or not. All the fans in the fan house broke.
It'd be nice if all usage of reserved-words was strictly forbidden and asserted on by the engine, because we've had several problems with this which have been messy to track down.
Personally, this problem is usually introduced by yours truly forgetting to query or set 'vars.active' instead of 'active' in one or two places. I'm not really sure how we could catch this case. :/
I'm asking that vars.active be illegal - i.e. that reserved words, which are now legal for use in vars/properties when properly prefixed, become illegal. Because they tend to just be a rats-nest of problems, and it's not hard to use something else.
I'd second this. I've had problems caused by it.
Has been recently fixed as part of the strict mode conversion - strict mode won't let you use a property name that collides with the name of a built-in.