I found this in the 6.5.28 release notes
4.4 Operating System
4.4.1 Bugs fixed in IRIX 6.5.28
+ 919019: ILLEGAL_OUT_FORMAT error on glDrawPixels
Unfortunately, the Xscreensaver error still occurs under Vpro in 6.5.28.
Jan 11 08:52:14 4A:holmes4 unix: WARNING: odsy board 0: ILLEGAL_OUT_FORMAT received
Jan 11 08:52:14 1A:holmes4 unix: ALERT: odsy board 0: Graphics error
Jan 11 08:52:14 7A:holmes4 unix: odsy flags: 0xffffffff82001100<GEN_LOCK_INTR,CFIFO_HW_FLAG,CTXSW_DONE,ILLEGAL_OUT_FORMAT>
Jan 11 08:52:14 7A:holmes4 unix: odsy status0: 0x10024280<CFIFO_ENABLED,CFIFO_LW,XRFIFO_LW,RASTER_SYNC_SRC=unset,CFIFO_SYNC_SRC=unset,DMA_SYNC_SRC=unset>
Jan 11 08:52:24 1A:holmes4 unix: ALERT: RAD thread held off for more than 32 mS
I grep hunted all of the Xscreensaver source for glDrawPixels but was surprised to find no occurances.
Still a bug lunking in the IRIX video drivers?