I MADE BUTTERCREAM FROSTING TODAY FOR A 3D HELMET CAKE.  THE FROSTING HAD LUMPS OF POWDER SUGAR IN IT EVEN THOUGH I THOUGHT I MIXED LONG ENOUGH..  IT WAS A BUTTER, POWDER SUGAR AND WATER RECIPE.   WHEN I APPLIED IT, IT PULLED OFF MY CRUMB COAT AND WAS "PIECEY".   I HAD TO GO BACK OVER WITH A HOT SPATULA TO SMOOTH ON.  I HOPE MY FONDANT DOESNT SLIDE OFF OVERNIGHT!!!!!!!!!

WHAT DID I DO WRONG?

I ALSO SIFTED THE SUGAR

I whip the crap out of my buttercream, I just whip it til it looks creamy. It also sounds like it might have been a little thick, add more water or milk (by the teaspoon) unitl you get the consistency you want. Did you use salt?

ANo salt . it was really nice and creamy when I first took it out of the mixer but after about 5 minutes of spreading it on it would get lumpy and just fall off the cake in pieces. I wet the frosty a tad bit before putting the fondant on but I still had a hard time getting the fondant to stick to it

If you read through my recipe "2 of everything icing" you will see that mixing it for as much as 10 minutes will result in a nice smooth, creamy icing.  Most people only mix it 'until incorporated' or for 2 or 3 minutes but letting that mixer go on slow speed for a much longer time will result in a much better icing.

Also the fact that you had lumps of butter &/or powdered sugar proves it was not mixed enough.  Also because you say it pulled away from the cake that tells me the consistency was too thick.  It needed to be thinned out - by additional liquid &/or more beating.

All b'cream icings need to be remixed after sitting unused for as little as 10 minutes or it will develop air bubbles.