Yep! Turn that mixer on and go watch an espisode of Seinfeld or something. 
(I used to turn on my mixer, then walk next door to the Dollar General store to visit with the girls over there, or walk to the Burger King on my parking lot and get lunch, have a smoke break and by the time I was back, I had perfect icing! 
)
If beating it for a few more minutes doesn't help, dip the mixing bowl bottom in warm water for a minute and stir until they start to melt again and then put it back on the mixer. It happens. 
This happened to me a few times. the first was when i was making a wedding cake. When it happened again a few days later the only thing i had done different was use margarine that was not refrigerated and was very soft. Never used butter that had not been refrigerated again and never had the problem again. I've been making cakes for over 30 years.
Debp
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