I am trying to make a video out of several files. As is often the
case, the different parts of the video overlap in time, but some
finish before others.

I know a bug report should be self-contained, but OTOH I cannot paste
here >300MB of data for you to reproduce this bug, so I'll leave the
needed files available at:

https://nube.gwolf.org/public.php?service=files&t=368628c8829be0303e6f2444e09860ef

I try to render the "Chema_Serralde_todo_webm.xptv" file with the
following settings:

- General
  - Preset: HTML5
  - Container format: WebM
- Video:
  - Scale 100% (640x480)
  - Frame rate:23.976 fps
  - Codec: On2 VP8
- Audio:
  - Channels: mono
  - Sample rate: 44.1KHz
  - Sample depth: 16 bit
  - Codec: Vorbis

The render starts (although the target file gets generated, but stays
at 0). However, when the render reaches 20% (0:28:58.203, that is,
when the clip P3190002.webm finishes), rendering just stops advancing,
and the preview window stays still. I get the following messages sent
to the invoking console:

ERROR [ 6877] [0x7f7e757e3700] "<Pipeline at 0x7f7e5a414e50>"   pipeline          Mar 27 12:31:59      _handleErrorMessage: error from /GstPipeline:pipeline0/GstBin:bin0/GnlComposition:gnlcomposition0/GnlSource:gnlsource: FileSourceFactory3/GstBin:bin6/pitivi+elements+singledecodebin+SingleDecodeBin:pitivi+elements+singledecodebin+singledecodebin3/GstMatroskaDemux:matroskademux3 (__main__.GstMatroskaDemux): GStreamer encountered a general stream error. (matroska-demux.c(4492): gst_matroska_demux_loop (): /GstPipeline:pipeline0/GstBin:bin0/GnlComposition:gnlcomposition0/GnlSource:gnlsource: FileSourceFactory3/GstBin:bin6/pitivi+elements+singledecodebin+SingleDecodeBin:pitivi+elements+singledecodebin+singledecodebin3/GstMatroskaDemux:matroskademux3:
stream stopped, reason not-negotiated) (/usr/lib/pitivi/python/pitivi/pipeline.py:858)
Traceback (most recent call last):
  File "/usr/lib/pitivi/python/pitivi/pipeline.py", line 918, in _binPadAddedCb
    handled |= action.handleNewStream(factory, stream)
  File "/usr/lib/pitivi/python/pitivi/action.py", line 502, in handleNewStream
    prodstream, consstream, init=False)
  File "/usr/lib/pitivi/python/pitivi/action.py", line 640, in _activateLink
    tee.link(queue)
gst.LinkError: failed to link tee5 with queue36
ERROR [ 6877] [0x7f7e757e3700] "<Pipeline at 0x7f7e5a414e50>"   pipeline          Mar 27 12:31:59      _handleErrorMessage: error from /GstPipeline:pipeline0/GstBin:bin0/GnlComposition:gnlcomposition0/GnlSource:gnlsource: FileSourceFactory0/GstBin:bin1/pitivi+elements+singledecodebin+SingleDecodeBin:pitivi+elements+singledecodebin+singledecodebin0/GstQTDemux:qtdemux17 (__main__.GstQTDemux): GStreamer encountered a general stream error. (qtdemux.c(3891): gst_qtdemux_loop (): /GstPipeline:pipeline0/GstBin:bin0/GnlComposition:gnlcomposition0/GnlSource:gnlsource: FileSourceFactory0/GstBin:bin1/pitivi+elements+singledecodebin+SingleDecodeBin:pitivi+elements+singledecodebin+singledecodebin0/GstQTDemux:qtdemux17:
streaming stopped, reason not-negotiated) (/usr/lib/pitivi/python/pitivi/pipeline.py:858)

Thanks a lot for any help solving this issue.

The example files were gone, but this should work now with Pitivi
0.93-4.2 in jessie. I tried with three overlapping files (mixed webm and
mp4) and outputted with your settings successfully.