This happened just last week!! Was in a hurry and did not wait for the meringue to cool down before putting in the butter!!!In the end, i get a soupy and runny butter cream coz the butter was melted!! Did'nt have the heart to discard the whole thing. so what i did, i put in cocoa and milk powder to alter the consistency: from soupy to firm!!! Not bad at all. Anyway, learn my lesson, indeed!
This has happened to me before too, and I brought it back to a good consistency by packing bags of frozen vegetables around the mixer bowl while it was whipping. It takes a Loooong time, but it will eventually come back if you keep it cold enough. If I remember correctly, I think that I've also read that people have put the bowl into the fridge until it cools down and whipped it then, and that worked, too.