it kicks in at the :VARARG position.
Good point, thanks

Is there any "mixed" WinAPI call that you know of? There are more than 1,000 VARARG API calls in \Masm32\include...
Just for fun, I checked it (source & exe attached):
include \masm32\MasmBasic\MasmBasic.inc
SetGlobals ctVar, ctMixed
Init
GetFiles "\Masm32\include\*.inc"
PrintLine Str$("%i files found\n", eax)
For_ fcount=0 To Files$(?)-1
Recall Files$(fcount), L$()
For_ ct=0 To eax-1
mov esi, L$(ct)
.if Instr_(esi, ":VARARG")
inc ctVar
lea ecx, [edx+6] ; +6 for len(":VARARG")
.if Len(esi)!=ecx
inc ctMixed
Print Str$("\n%___i", ct), esi
.endif
.endif
Next
Next
Inkey Str$("\n%i vararg calls found", ctVar), Str$(", of which %i are mixed", ctMixed)
EndOfCode