As posted by Demonstar55 a while ago, there are client-side spell ID limits, why? Who knows, but here is a list.

Client Max Spell ID
Titanium 9,999
Secrets of Faydwer 15,999
Seeds of Destruction 23,000
Underfoot 28,000
Rain of Fear


Rain of Fear 2 45,000
The Darkend Sea (unsupported) 50,000