In Gen 4, the combination Klutz + Iron Ball is not properly handled about damage-calc HOT 5 CLOSED

What does it look like when you remove the Klutz ability?

from damage-calc.

The Iron Ball speed drop is properly applied.

from damage-calc.

I suspect the issue comes from the fact that checkItem gets rid of the item entirely if Klutz is the Ability. Since checkItem is common to all generations, one might want to keep track of the old item near the start of calculateDPP and compare with it on this line.

from damage-calc.

That line appears to deal with the grounding effect of Iron Ball, which according to Bulbapedia, gets ignored by Klutz. In other words, Klutz allows the speed to be halved still, but doesn't allow the Pokemon to be grounded. However, this line and this line might have something to do with how the damage calc handles the speed of the Pokemon.

from damage-calc.

You're right, i linked you to a completely wrong reference, sorry about this.
Your second link is the checkItem function that is common to all generations. Probably not being modified, due to possible side effects in the other calculators split per generation.
Your first link is indeed where the speed drop is supposed to be applied, but since checkItem was already called previously and discards the item even in the case of an Iron Ball, the item is gone. Thus the drop is not put inside speedMods.

from damage-calc.