so yeah, my camera makes broken avi videos and I had a bunch of them to repair :

for i in *.avi; do mencoder -idx $i -ovc copy -oac copy -o $i.repaired.avi; done

( that’s a command-line for those who wonder )

this only repairs the index, no re-encoding

divfixpp repairs indexes as well, but I didn’t try it

hope it helps