Forums Scripts & Popups on quit

Well, in fact, there is a slight difference:
ison checks for: "nick v1 is a nick in the nicklist of v2". (Therefore, v1 being a user on that chan, and v2 being a valid chan).
ischan checks for: "v1 being a open channel window". This is more precise than the quoted "is a channel which you are on" -

To see what I mean:
- disable the "rejoin channel when kicked" or "rejoin channels on connect" option. have "keep channels open" enabled.
- prior to being kicked or disconnected, both statements will be true: "$me ison #chan" and "#chan ischan".
- get kicked out of that chan or disconnect.
- now (disconnected or kicked) "$me ison #chan" will be false, but "#chan ischan" still true: as that channel window remained open.

Imho, the condition "ischan" is completely sufficient in most circumstances.