In the SNES version (no idea about the GBA version), losing an animal friend by crossing a sign counts as defeating an enemy. This is useful in Bash the Baddies style bonus stages with an animal friend and a No Animal Sign at the end. In these stages, you can miss one enemy and you can still get the Kremkoin. One place that you can do this is the bonus in Target Terror where you use Squawks to defeat a bunch of Zingers in a bramble setting. Another place where this is possible is one of the bonuses where you are Enguarde and you have to beat a lot of Flotsams and Puftups -- I think this bonus was in Lava Lagoon, but I'm not sure. There are probably other places where you can do this too. This glitch is useful in speedruns and if you're lazy!
Oh, and it doesn't work in Donkey Kong Land 2 -- I tried.