Call of Duty 5: Fixing Exceeded 400Fxs assets
From COD Modding & Mapping Wiki
Make sure to comment out or remove the line impactfx,mp_yourmap from your Zone File to fix
Also see HERE
“Try removing the "impactfx" line from your zone source file ( .csv ) and see if the problem with the FX goes away.
This seems like a bug with the launcher placing this line in there automatically, when it shouldn't.”
- Chris P., Treyarch Staff at Modsonline.com