Try this:

gst-launch videotestsrc num-buffers=500 ! capsfilter caps="video/x-raw-rgb,width=1000,height=800" ! videorate ! ffmpegcolorspace ! timeoverlay ! ffmpegcolorspace ! ffenc_huffyuv ! avimux ! filesink location="test.avi"

You will get this:

0:00:01.039154423 25249  0x80934b0 ERROR               ffmpeg :0:: encoded frame too large
0:00:01.039178527 25249  0x80934b0 ERROR               ffmpeg :0:: encoded frame too large
0:00:01.039202958 25249  0x80934b0 ERROR               ffmpeg :0:: encoded frame too large
0:00:01.039227194 25249  0x80934b0 ERROR               ffmpeg :0:: encoded frame too large
0:00:01.039251448 25249  0x80934b0 ERROR               ffmpeg :0:: encoded frame too large
0:00:01.039275572 25249  0x80934b0 ERROR               ffmpeg :0:: encoded frame too large
0:00:01.039299877 25249  0x80934b0 ERROR               ffmpeg :0:: encoded frame too large
....

The resulting file (test.avi) will be corrupt. (At least no media player I've tried can play it back.)

Lower resultions, for example 640x480, is OK. 

Since the only available lossless video encoders are ffenc_ffv1, ffenc_huffyuv and ffenc_ffvhuff where ffenc_ffv1 and ffenc_ffvhuff seems totally broken, gstreamer effectively has NO lossless encoder for HDTV resolutions.