This has not happened to me for a long time so I thought I would share. I am making a 3 gallon batch of my simple Welch's wine and today has been a couple days since I pitched yeast. Fermentation was rolling strong but there was no real foam and such on top. I figured it was calmed down enough that I could top up the carboy with the liter I siphoned off before pitching yeast. I was wrong! An hour later something happened and there was wine all over the counter and an airlock flown across the room. Hope my wife does not mind the smell I can not seem to get rid of now.