Comment 3 for bug 386397

I am experiencing the same problem.

It seems to be related to the architecture and mjpeg handling.

I could transform from h264 avi to flv without a problem, but when trying to extract a frame as jpeg I also got the "Illegal instruction" error:
ffmpeg -i Fenix.flv -t 00:00:1 -f mjpeg Fenix.jpg
FFmpeg version 0.5-svn17737+3:0.svn20090303-1ubuntu6, Copyright (c) 2000-2009 Fabrice Bellard, et al.
  configuration: --enable-gpl --enable-postproc --enable-swscale --enable-x11grab --extra-version=svn17737+3:0.svn20090303-1ubuntu6 --prefix=/usr --enable-avfilter --enable-avfilter-lavf --enable-libgsm --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libvorbis --enable-pthreads --disable-stripping --disable-vhook --enable-libdc1394 --disable-armv5te --disable-armv6 --disable-armv6t2 --disable-armvfp --disable-neon --disable-altivec --disable-vis --enable-shared --disable-static
  libavutil 49.15. 0 / 49.15. 0
  libavcodec 52.20. 0 / 52.20. 0
  libavformat 52.31. 0 / 52.31. 0
  libavdevice 52. 1. 0 / 52. 1. 0
  libavfilter 0. 4. 0 / 0. 4. 0
  libswscale 0. 7. 1 / 0. 7. 1
  libpostproc 51. 2. 0 / 51. 2. 0
  built on Apr 10 2009 23:18:41, gcc: 4.3.3

Seems stream 0 codec frame rate differs from container frame rate: 1000.00 (1000/1) -> 50.42 (605/12)
Input #0, flv, from 'Fenix.flv':
  Duration: 00:00:33.30, start: 0.000000, bitrate: 1000 kb/s
    Stream #0.0: Video: flv, yuv420p, 640x480, 1000 kb/s, 50.42 tbr, 1k tbn, 1k tbc
Output #0, mjpeg, to 'Fenix.jpg':
    Stream #0.0: Video: mjpeg, yuvj420p, 640x480, q=2-31, 200 kb/s, 90k tbn, 50.42 tbc
Stream mapping:
  Stream #0.0 -> #0.0
Press [q] to stop encoding
Illegal instruction

Assuming Josh's backtrace is a proper one (never done one before) here is my output after press [q]...:
[New Thread 0xb6e096c0 (LWP 4504)]

Program received signal SIGILL, Illegal instruction.
[Switching to Thread 0xb6e096c0 (LWP 4504)]
0xb802a714 in ?? () from /usr/lib/libswscale.so.0

and /proc/cpuinfo

processor : 0
vendor_id : GenuineIntel
cpu family : 5
model : 4
model name : Pentium MMX
stepping : 3
cpu MHz : 233.859
cache size : 0 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : yes
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 1
wp : yes
flags : fpu vme de pse tsc msr mce cx8 mmx up
bogomips : 467.71
clflush size : 32
power management:

Running the same command on the same file on my amd 64x x2 works as it should.