This is the sample file I used in this issue:
https://roundup.ffmpeg.org/roundup/ffmpeg/issue872

=====
I try to transcode a flv file to mp4 using libx264, but encountered this error
message "av_interleaved_write_frame(): Error while opening file".  The command
line I use is:

$ffmpeg -i video.flv -vcodec libx264 video.mp4

The output is:
FFmpeg version SVN-r17630, Copyright (c) 2000-2009 Fabrice Bellard, et al.
  configuration: --enable-gpl --enable-libx264 --enable-libfaac --enable-libfaad
--enable-libmp3lame
  libavutil     49.15. 0 / 49.15. 0
  libavcodec    52.20. 0 / 52.20. 0
  libavformat   52.30. 1 / 52.30. 1
  libavdevice   52. 1. 0 / 52. 1. 0
  built on Feb 27 2009 17:19:01, gcc: 4.0.1 (Apple Inc. build 5490)

Seems stream 0 codec frame rate differs from container frame rate: 1000.00
(1000/1) -> 30.00 (30/1)
Input #0, flv, from 'video.flv':
  Duration: 00:05:38.30, start: 0.000000, bitrate: 361 kb/s
    Stream #0.0: Video: flv, yuv420p, 320x240, 297 kb/s, 30 tbr, 1k tbn, 1k tbc
    Stream #0.1: Audio: mp3, 22050 Hz, mono, s16, 64 kb/s
File 'video.mp4' already exists. Overwrite ? [y/N] y
Output #0, mp4, to 'video.mp4':
    Stream #0.0: Video: libx264, yuv420p, 320x240, q=2-31, 200 kb/s, 90k tbn, 30 tbc
    Stream #0.1: Audio: libfaac, 22050 Hz, mono, s16, 64 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #0.1 -> #0.1
[libx264 @ 0x1008a00]using cpu capabilities: MMX2 SSE2Fast SSSE3 PHADD SSE4.1
Cache64
[libx264 @ 0x1008a00]profile Baseline, level 1.3
Press [q] to stop encoding
[libx264 @ 0x1008a00]error, non monotone timestamps 488 >= 488 429.7kbits/s    
av_interleaved_write_frame(): Error while opening file

My investigation showed me that the cause was introduced in r17555:

[ybian@wukong:ybian/src/ffmpeg-svn]$ svn log -r 17555
------------------------------------------------------------------------
r17555 | michael | 2009-02-24 23:06:17 +0800 (Tue, 24 Feb 2009) | 4 lines

Do not duplicate frames if the muxer claims that it is not needed.
Note to release manager, this is risky and should be reverted if
it causes any problem.

------------------------------------------------------------------------

[ybian@wukong:ybian/src/ffmpeg-svn]$ svn diff -r 17555:17554
Index: ffmpeg.c
===================================================================
--- ffmpeg.c    (revision 17555)
+++ ffmpeg.c    (revision 17554)
@@ -865,7 +865,7 @@
         //FIXME set to 0.5 after we fix some dts/pts bugs like in avidec.c
         if (vdelta < -1.1)
             nb_frames = 0;
-        else if (video_sync_method == 2 || (video_sync_method<0 &&
(s->oformat->flags & AVFMT_VARIABLE_FPS)))
+        else if (video_sync_method == 2)
             ost->sync_opts= lrintf(get_sync_ipts(ost) / av_q2d(enc->time_base));
         else if (vdelta > 1.1)
             nb_frames = lrintf(vdelta);


If I reverted the change in revision 17555, the problem disappeared.