global enchantments when there are 5 already?

[Crazy]

Here is the scenario:

There were 4 enchantments and I cast one.

Next turn. An opponent casts a global enchantment which takes effect(5 total now). I get a message saying that my enchantment is cast but when I check it is not there.

If there are 5 and an enchantment is cast, is there a dispel attempt for one of the 5 current globals or is one of the 5 randomly replaced by the new one being cast??

Thanks.