I own two S60 phones, a 7610 and a 6680. Recently I tried this thing on an N91 and experienced the same strange behaviour.
1) Start a new text message
2) Write something
3) Press the 0 key three times to see the arrow indicating a new line will be started (it is also the last symbol on the symbols page)
4) Immediately, BEFORE THE ARROW SYMBOL DISAPPEARS, press a key. The following strange behaviours are observed:
If you press the Options soft key, the first menu item "Add Recipient" is chosen
If you press the UP direction key, the new line is inserted into the To box (or whatever line is above what you wrote)
If you press the LEFT direction key, the new line behaves normally
If you press the RIGHT direction key when the cursor is at the end of the text, the messaging app crashes instantly
If you press the RIGHT direction key when the cursor is at the beginning/in the middle of of a word, a newline is inserted 1 character after the cursor position
If you press the DOWN direction key when there are no lines below the line where the cursor is, the newline is inserted at the end of the text
If you press the DOWN direction key when there are lines below the line of the cursor, the newline is inserted on the line below, possibly splitting a word
Of course, the most annoying thing is the messaging app crashing destroying what the user has written if 0, 0, 0, RIGHT is pressed.
Can anyone else try this on their S60 and see if it's just my bad luck?