Line 882 of ssh-agent.c reads:
if (ac == 0 && !c_flag && !k_flag && !s_flag && !d_flag) {
   stuff to set c_flag=1 iff $SHELL=*csh
}

This means that someone running ssh-agent -k will have k_flag set and therefore 
won't get the auto-shell determination, therefore leaving them with bash-style 
commands.

I'm guessing the debug flag shouldn't be here either, so the correct line would 
read:
if (ac == 0 && !c_flag && !s_flag) {

This was originally found to be broken on IRIX 6.5.15m, but looking at the 
source made it obvious it was cross-platform.