So I discovered these bugs lately...

Looks like the () around the if statement makes it execute the command after ) if it's true
//noop $iif((1 isnum) echo -ag fail,1)
returns (echoes):
fail /!return 1

And.. mIRC has troubles parsing:
//echo -a $replace(ab.,},}) . c
returns(echoes):
abc

Your $iif() has a bad syntax. $iif((1 isnum), echo -ag fail,echo 1)
Also why do you need to noop it? I don't understand.

I am not sure if its a bug or not, } is kinda a reserved character. But replacing it from a text using replace w/o $eval() kinda should return such results.

Er, $iif can be used with 2 parameters, $iif(blah,do this), returns do this or $null depending if its true or false..

I know that already, but $iif((1 isnum) echo -ag fail,1) <== doesn't look right.

But even if it's wrong syntax, mirc should not behave this way, and the bug with { } is as odd as $eval({,1) $eval({,2} etc... or "//echo -a ok } echo -a ok2" and such

There is no 'but'!
A wrong syntax will create wrong results.

it's your opinion, mine is that usually, mirc try to prevent wrong syntax to be executed.

Sephiroth_ is right, and also the syntax looks rather exploited than wrong to me. The wrong result is expected.

Whether it looks right or not, there's nothing syntatically invalid with that code. There's no reason that code shouldn't work.