We thought it was the bitrate at the time, but it turned out it was the pulldown flag. The new segment had been encoded as 23.xxx w/o the pulldown flag, while the surrounding clips had the flag, which Womble interprets as 29.xxx. I've mixed bitrates for some time since with no problem, so long as the maximum doesn't exceed the maximum allowed.

SW had 9800 bitrates on the video and the most I could get in TMPGEnc was 8000 anyway, so I couldn't break the ceiling if I wanted to. All the Supermans have their video at 7500, incidentally. Any clips I've done for the purposes of FX have been at 7999 or 8000 which has worked fine.