The option "Show Notices in Active Window" clashes with Network Operators' Server Notice (SNOTICE, /help on SNOTICE) messages.

While technically still a notice, they convey different things entirely: Server Notices are server-related messages, whereas notices mostly are people manually typing /notice, or messages with NickServ. The main difference here is: One thing is useful (normal notices), the other is just noise (server notices).

Considering the dialogue option says "notices" and not "notices and snotices", I'm going to assume this to be a misbehaving feature. Considering they are also separate events and ON NOTICE does not respond to ON SNOTICE.

There may be some discussion about whether this is a feature suggestion or a bug report, all in all I don't think it matters a lot.

Workarounds for this problem do not work because of mIRC's stripping whitespace problem (where you disable the option and write your own), although this does:
on ^*:SNOTICE:*: {
  echo -tngs  $+ $color(notice) - $+ $nick $+ - $1-

I still however believe this is a wrongly behaving feature, and I think it would be a performance boost if I hadn't had to special-case every single Server Notice I receive.

Overall, the fix for this should be pretty small, and no UI changes would be required. People wanting to mimic the behavior for the current could run on `*:SNOTICE:*: { echo -a ... }` instead, although only madmen would do this considering it generates a lot of noise.