I made my first wedding cake about a week ago. It had three chocolate tiers each stacked directly on top of the other; no pillars. I used Duncan Hines cake mixes and the bottom layers turned out fine, but the top layer slowly leaked a sticky, reddish fluid from the time it was frosted until the time it was eaten!
Thankfully, after I actually had the cake together, the border around the base of the top layer acted as a dam to keep it from running down the sides of the cake, but when I lifted the top layer off to cut the cake, there was a ring of the nasty stuff underneath it.
What causes this? And why did only one of the three cakes have this affliction since (I think) I made them all the same way?

Did it have a filling? I have had a cake where the dam was not big enough or I just over filled the filling and that causes nasty leakage.

It was frosted and filled with white buttercream.

I just had the same problem with a cake for a wedding I did yesterday. It did not show up until the cake was totally frosted and taken to the wedding for setup.

I have no idea if this could be the problem but I had a sweaty, leaky cake because I frosted too soon. If the cake isn't absolutely cool it sweats under there. But I have also heard on here that if you refrigerate the cake and then bring it out in the warm air it can get runny/sweaty. Sorry, probably not much help.