I posted about this two days ago:
http://www.cybertron.ca/boards/showthread.php?t=72931
and WereDragon linked to the first time it was mentioned in the Alberta Sightings thread as well.
It's the blue pegs that connect the back of his knees (ish) to the outer "shell" of the "boot"
When converting to shark mode, those pegs go into a certain groove in the shell - but the groove is not deep enough, so the pegs don't go in all the way, which prevents the shell from tabbing properly.
I had success shaving off a millimetre or two off the pegs with an exacto-knife, holds together fine in both modes