Why did I get red flashes - single frames of red - in a Media Encoder output?

Hi everyone,

I have found that, at least from projects with enough effects in them, that outputting from Premiere to H.264 has been pretty treacherous.  Especially when I'm on a deadline, I'm looking for ways to reduce risk.

I recently took the following approach:  I output all 3 of my timelines for a project to Quicktime ProRes 422. All three ProRes movies came out clean and flawless.

Then I dropped those 3 separate Quicktime ProRes movies into a Media Encoder Queue to encode to H.264.  Generally, I'd hope that by baking my projects into a single Quicktime movie, I can sidestep many pitfalls (memory issues, troublesome effects, etc.)

Two of my 3 ProRes movies encoded just fine, producing clean H.264s for internet posting.  However, the third movie rendered out with two red flashes, single frames of red, which are NOT present in the ProRes originals (I double-checked.)

The 3 movies are very similar in length, content, etc. because they are 3 versions of the same program.

Again, looking back on the same frames on the original ProRes movie, there is no flash or glitch, nor anything else out of the ordinary.  So 2:29:06 in the H.264 is a frame of red, going back to 2:29:06 on the original - no red flash, nothing.

I rendered them all together, in the same queue.  I was in a hurry.  We video editors often are.  I am not able to find a record of what order the movies encoded in - was the one that came out with red flashes first, second or third?

So I'm very puzzled by why this would happen in the first place, and even so, why selectively?

My setup:  Late 2013 MacPro, 64 GB RAM, Mac OS 10.11.6, CC 2017.0.2 release (not the most current one.)  Media Encoder 11.02.53.

Thoughts?