How to deal with "buffer queue overflow, reset". (Ffmpeg)

Sometimes it works fine and sometimes I get this message [Parsed_overlay_0 @ 0x7f9520518800] [framesync @ 0x7f9521021e28] Buffer queue overflow, dropping.

by running this command ffmpeg -i a1.mp4 -i watermark150x125.png -filter_complex 'overlay=10:10' output.mp4

I tried different options, "piggybacking", for example overlay=W-w-5:H-h-5

, overlay=10:main_h-overlay_h-10

, overlay=main_w-overlay_w-10:main_h-overlay_h-10

- obtaining the same result.

Here is the output of the ffmpeg command.

ffmpeg version 2.6.2 Copyright (c) 2000-2015 the FFmpeg developers
  built with Apple LLVM version 6.1.0 (clang-602.0.49) (based on LLVM 3.6.0svn)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/2.6.2 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-libx264 --enable-libmp3lame --enable-libvo-aacenc --enable-libxvid --enable-vda
  libavutil      54. 20.100 / 54. 20.100
  libavcodec     56. 26.100 / 56. 26.100
  libavformat    56. 25.101 / 56. 25.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 11.102 /  5. 11.102
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f952101d200] stream 0, timescale not set
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'a1.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    artist          : a1
    album           :
    comment         : 
    date            : 0
    genre           :
    title           : a1
    encoder         : Lavf56.4.101
  Duration: 00:01:41.51, start: 0.000000, bitrate: 227 kb/s
    Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 426x240 [SAR 1:1 DAR 71:40], 97 kb/s, 29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 125 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
    Stream #0:2: Video: mjpeg, yuvj420p(pc, bt470bg/unknown/unknown), 120x90 [SAR 1:1 DAR 4:3], 90k tbr, 90k tbn, 90k tbc
Input #1, png_pipe, from 'watermark150x125.png':
  Duration: N/A, bitrate: N/A
    Stream #1:0: Video: png, rgba, 150x125 [SAR 2835:2835 DAR 6:5], 25 tbr, 25 tbn, 25 tbc
[swscaler @ 0x7f9521013e00] deprecated pixel format used, make sure you did set range correctly
[libx264 @ 0x7f9521030e00] using SAR=1/1
[libx264 @ 0x7f9521030e00] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 AVX2 LZCNT BMI2
[libx264 @ 0x7f9521030e00] profile High, level 2.1
[libx264 @ 0x7f9521030e00] 264 - core 144 r2533 c8a773e - H.264/MPEG-4 AVC codec - Copyleft 2003-2015 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'output.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    artist          : a1
    album           :
    comment         : 
    date            : 0
    genre           :
    title           : a1
    encoder         : Lavf56.25.101
    Stream #0:0: Video: h264 (libx264) ([33][0][0][0] / 0x0021), yuv420p, 426x240 [SAR 1:1 DAR 71:40], q=-1--1, 29.97 fps, 30k tbn, 29.97 tbc (default)
    Metadata:
      encoder         : Lavc56.26.100 libx264
    Stream #0:1(und): Audio: aac (libvo_aacenc) ([64][0][0][0] / 0x0040), 44100 Hz, stereo, s16, 128 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
      encoder         : Lavc56.26.100 libvo_aacenc
Stream mapping:
  Stream #0:0 (h264) -> overlay:main (graph 0)
  Stream #0:2 (mjpeg) -> overlay:overlay (graph 0)
  overlay (graph 0) -> Stream #0:0 (libx264)
  Stream #0:1 -> #0:1 (aac (native) -> aac (libvo_aacenc))
Press [q] to stop, [?] for help
[Parsed_overlay_0 @ 0x7f9520518800] [framesync @ 0x7f9521021e28] Buffer queue overflow, dropping.
    Last message repeated 385 times
[Parsed_overlay_0 @ 0x7f9520518800] [framesync @ 0x7f9521021e28] Buffer queue overflow, dropping.
    Last message repeated 564 times
[Parsed_overlay_0 @ 0x7f9520518800] [framesync @ 0x7f9521021e28] Buffer queue overflow, dropping.
    Last message repeated 546 times
[Parsed_overlay_0 @ 0x7f9520518800] [framesync @ 0x7f9521021e28] Buffer queue overflow, dropping.
    Last message repeated 569 times
[Parsed_overlay_0 @ 0x7f9520518800] [framesync @ 0x7f9521021e28] Buffer queue overflow, dropping.
    Last message repeated 593 times
[Parsed_overlay_0 @ 0x7f9520518800] [framesync @ 0x7f9521021e28] Buffer queue overflow, dropping.
    Last message repeated 312 times
frame= 3041 fps=645 q=-1.0 Lsize=    1800kB time=00:01:41.52 bitrate= 145.3kbits/s dup=2975 drop=0
video:149kB audio:1587kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 3.700745%
[libx264 @ 0x7f9521030e00] frame I:14    Avg QP:19.91  size:  6947
[libx264 @ 0x7f9521030e00] frame P:768   Avg QP:10.30  size:    27
[libx264 @ 0x7f9521030e00] frame B:2259  Avg QP:13.67  size:    15
[libx264 @ 0x7f9521030e00] consecutive B-frames:  0.9%  0.1%  0.1% 98.9%
[libx264 @ 0x7f9521030e00] mb I  I16..4: 18.7% 59.0% 22.3%
[libx264 @ 0x7f9521030e00] mb P  I16..4:  0.0%  0.0%  0.0%  P16..4:  0.1%  0.0%  0.0%  0.0%  0.0%    skip:99.9%
[libx264 @ 0x7f9521030e00] mb B  I16..4:  0.0%  0.0%  0.0%  B16..8:  0.1%  0.0%  0.0%  direct: 0.0%  skip:99.9%  L0:50.7% L1:49.3% BI: 0.0%
[libx264 @ 0x7f9521030e00] 8x8 transform intra:59.1% inter:31.9%
[libx264 @ 0x7f9521030e00] coded y,uvDC,uvAC intra: 21.4% 12.4% 7.4% inter: 0.0% 0.0% 0.0%
[libx264 @ 0x7f9521030e00] i16 v,h,dc,p: 39% 30% 30%  0%
[libx264 @ 0x7f9521030e00] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 34% 47% 16%  1%  0%  0%  1%  0%  1%
[libx264 @ 0x7f9521030e00] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 20% 38% 15%  3%  3%  3%  6%  4%  7%
[libx264 @ 0x7f9521030e00] i8c dc,h,v,p: 80% 15%  4%  1%
[libx264 @ 0x7f9521030e00] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x7f9521030e00] ref P L0: 96.5%  1.2%  2.3%
[libx264 @ 0x7f9521030e00] ref B L0: 51.5% 48.5%
[libx264 @ 0x7f9521030e00] ref B L1: 99.6%  0.4%
[libx264 @ 0x7f9521030e00] kb/s:11.97