According to my understanding, it's currently not possible to drop an index on a collection at the same time that a separate index is in the process of being built on the same collection. As of SERVER-32033 you can call dropIndex on the in-progress index and the build will be cancelled, but if you try to drop a different already built index you'll get a BackgroundOperationInProgressForNamespace error. I'm guessing this comes from this check here.

I think this behavior is confusing (why should building index A interfere with dropping index B?) and makes it more difficult to manage indexes.