Make sure you did not get the DK coin on the bobbing barrels level!!
If you got it, make a new file and get to the end of the 2nd world.
Find the koindozer in the barrel jumping level. Then, find the TNT barrel and go to the koindozer. Bump into him and the blue balloon will be revealed, but with a redish and grayish color. There is no negative effects on the rest of the game, but it is a funny issue they didn't fix.
Submitted by: Copper20