I read that last bit in Hank Hill's voice.

Of course there is. A lot of older TV shows are effectively variable frame rates when properly detelecined/deinterlaced. And mixing two fixed frame rate sources with different framerates would naturally produce a VFR video and with current editing software you need to destroy some of the source information or interpolate some slop to make it work. There is no good reason for editing software to have these limitations either when most players and all modern video containers have caught up and are VFR-first with a fixed frame rate hint in the headers if you're lucky.