Workaround using try..catch

                  try {
                    sourceBuffer.timestampOffset += sourceBuffer.buffered.end(0);
                    resolveUpdatedMediaSource();
                  } catch (err) {
                    console.error(err);
                    resolveUpdatedMediaSource();
                  }

https://github.com/guest271314/recordMediaFragments/commit/da902dc083de8229f335cb057bea90407c5f30a7

Assigning to wolenetz@ from Issue 737757 for more inputs and further triaging of this.

This is very likely per spec. Without a specific repro to investigate, I cannot tell more certainly.

MSE spec text (within https://www.w3.org/TR/media-source/#dom-sourcebuffer-timestampoffset):
"If the append state equals PARSING_MEDIA_SEGMENT, then throw an InvalidStateError and abort these steps."

Specifically, even though the user agent has signalled 'updateend', it doesn't mean that the JS app ended that previous appendBuffer with bytes corresponding to the completion of the media segment.

What kind of bytestream are you using?
Is the app unable to determine the media segment boundary for that bytestream and adjust how it appends media and when it signals timestampOffset accordingly?
Note, if you need (though this may drop partially parsed but not yet fully buffered media in not-necessarily-interoperable ways across MSE implementations) to reset the parser state, there is the SourceBuffer.abort() operation which may assist your use case. Note that a future appendBuffer() following such an abort() on the SourceBuffer will require you begin at a segment (init or media) boundary for the bytestream.

Closing this as WAI. Please re-open if this diagnosis is incorrect (along with specific repro steps please).