i believe i have found a bug in $+ when an $iif statement is preceded by a null variable. example (make sure %test is unset or choose a different variable name):
//echo -a $+(%test,$iif(testing,test))
echos '$nulltest'. it doesnt matter how many parameters you pass to $+, nor does the position of the null variable\$iif combo in the list (i.e. $+(one,two,three,%var,$iif) breaks, as does $+(one,%test,two,$iif)).
one could argue that this is working correctly because %test is actually $null, but this is not true because the same statement switched around (or even with the $iif removed) excludes the $null.
i reproduced this bug on both mIRC 6.3 and 5.91