1) From the recent thread about editbox:
A difference between mIRC 6.35 and 7.32 is that using control + a + del to delete the whole content of the editbox and then using the up arrow key will show that line again on 7.32, while it will show the previous line on 6.35.
6.35's behavior is prefered imo.

2) When you enter multiple line of text in the automatic editbox without copy pasting and without reaching the end of the line (using alt + enter) mIRC doesn't see that as a block and store each line seperetaly:
-Type 'aaaaaaa' + alt + enter + 'bbbbbbbbbbbbbbbbbb...' and hit 'b' until you create a third lines, hit enter to makes sure mIRC register the lines (might want to try this in a non connected status window to avoid any trouble), hit the up arrow key and you'll get the last two lines, consisting of "bbbb..." the line with aaaaaaa has been registered seperatly and can be accessed with one more hit of the up arrow key.

Looking for a good help channel about mIRC? Check #mircscripting @ irc.swiftirc.net