We all have seen the warnings in lavatories...Flushing anything other than TP causes external leaks and safety hazard.

What EXACTLY does this mean? Please only answer is you really know the answer, and NOT guessing.

We've heard the horror stories about the toilet backing up, and this is a safety hazard. It's the "external leaks" part that has me puzzled. Sounds like they dump the toilets while mid-flight, which I know isn't true!