Made for the first time BC using 1/2 cup butter and 1/2 cup shortening, 1 1/2 tsp vanilla, 1/2 tsp Butter flavoring, 4 cups PS.
I had initially refrigerated the cake after frosting it, but then my husband left it out on the counter all day. We live in a cool place. I noticed that the frosting developed a little crunch to it, like the sugar crystalized. Dpes anyone know what I did wrong?
Thanks ahead of time for your help.

It sounds like it crusted.

My frostings crusts but it doesnt get crunchy, not even after it has been in the fridge, I wonder what happened...

Yes it did crust. Does that mean the entire frosting has a crunch to it though? I'm sure only a baker would notice, but I certainly did.

....crunch...crunch.

It was crunchy all the way through and not just the outer part? How long had it been sitting?

One time this happened to me but I noticed it before I put it on the cake. I was rushing & just threw all the ingredients in at the same time & just mixed it. Needless to say I did not get everything mixed well enough. There were little bitty balls of icing in the mix & it couldn't be smoothed, couldn't make roses, etc it was a mess
I didn't notice them until I added the color & once I put it in the bag to start the roses
I tried to re-beat it in the mixer & that didn't help I ending up throwing it out.

It didn't have balls that weren't mixed in, it was more like the sugar partially crystalized. I think it was throughout the entire frosting, but it was a bit hard to tell. It was still good, but not as smooth as it was initially.
I don't know how long it sat out of the frig, maybe 8 hours.