AOn friday I made and covered it with ganache. I put some fondant decorations on it and the next day the fondant started to melt into the ganache. Would this happen if I was to cover a cake with fondant? I really liked how smooth the cake looked but I'm afraid that this will happen again. I used two cups of chocolate chips to one cup of heavy cream, I let it set over night and used it friday. Did I do something wrong? I would appreciate any advice.

Was the cake refrigerated at any time?

AI put it in the freezer for about 10 minutes and took it out before I put the decorations on. Could have there still been some condensation?

Yes, absolutely. 

Did you also use water to attach the fondant decos?

No I didn't since it was just decorations and they were on the top of the cake.

Well, since ganache isn't a water based icing, the only thing that makes sense is condensation. 

The result would be at its worst if the cake was in an airtight plastic container and/or the room was very hot [over 80F) where it was stored.

Was the fonant hard when you put the decorations on? What was the weather like? What were the decorations like when you put them on?

Just as an aside, ingredients for ganache should be measured by weight, rather than by volume.

it was in the mid 70s that day, the fondant felt like a firm play dough. When I put them on they were fine, then in like 4 hours the fondant started to melt. I'm going to need to buy a scale then because I measured by volume.

D'oh! I meant ganache. Was the ganache hard when you put the decorations on? And when you say 'melt', do you mean the fondant softened, or that it got sticky, or that it was a puddle?

Did the fondant melt only where it touched the ganache, or all over?

The ganache was hard and when I say melt it got super sticky and it only happened where the letters where at.