UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/69.0.3497.100 Safari/537.36

Steps to reproduce the problem:
I'm using MediaRecorder API to record the speech on a web page. As an output, I'm getting the media chunks which represent parts of WebM audio.

Here is the code which is recording data:

`navigator.mediaDevices.getUserMedia({ audio: true, video: false }).then(function(stream) {
  recorder = new MediaRecorder(stream, { mimeType: 'audio/webm; codecs="opus"' })
  recorder.ondataavailable = function(e) {
    // Read blob from `e.data`, and send to sever;
  }
  recorder.start(1000)
})`

Those chunks cannot be played "as is", because they do not contain file headers. An only first chunk contains those headers, so if I want to play some part of the audio without combining the whole track, I'm concatenating the first chunk with the chunks I need to play and cut the audio with FFmpeg after. Like this:

`cat chunk_000.bin chunk_030.bin chunk_031.bin chunk_032.bin chunk_033.bin | ffmpeg -y -i - out.wav`

What is the expected behavior?
The expected behavior is that it does not matter which chunks we start with if the first chunk is attached

What went wrong?
But sometimes the data in those chunks is not playing after such concatenation (in the attached example it happens to chunk_059.bin file):

`cat chunk_000.bin chunk_059.bin | ffmpeg -y -i - out.wav`
This command gives an error:
[matroska,webm @ 0x2131100] Invalid track number 13569
[matroska,webm @ 0x2131100] Invalid stream 13569 or size 948

Though, if the problematic chunk appears in the middle of sequence - it works well:
`cat chunk_000.bin chunk_058.bin chunk_059.bin chunk_060.bin | ffmpeg -y -i - out.wav`

Did this work before? N/A 

Chrome version: 69.0.3497.100  Channel: stable
OS Version: OS X 10.13.6
Flash Version: 

Am I using this correctly? The "broken" chunks appear ~ once per 400 chunks?
 

Deleted: webm_parts.tar.gz 610 KB

webm_parts.tar.gz 610 KB Download