Zilean's Rewind ability, which lowers cd on his spells, does not queue. This creates problems when you're casting another spell and trying to lower its cd instantly, it's very easy to cast the cooldown before the spell finishes casting, which results in a wasted use of Rewind.

Steps to reproduce:

Step 1: Begin casting time bomb.
Step 2: Wait for time bomb to almost complete.
Step 3: Cast Rewind.
Step 4: Time bomb finishes casting, does not benefit from Rewind.
Step 5: Cry.