I have a sorted custom window where i regularly do /aline and /rline.
For some reason though, a linefeed was suddenly missing. It wasn't visible in the window, but only shows when I copied the data, also $line() showed the 2 merged lines as 1.
So i guess only internally the linefeed was gone.
Sadly I'm not able to reproduce this bug, also it happened on mIRC 6.32.

I've had the same issue recently: a quit and a join were merged onto the same line, in a normal channel window. This was with the default output, i.e. no scripts replacing the output. The timestamps showed that several hours had elapsed between the two events. Similarly, $line returned the concatenation of the two lines. Copying the line still yielded a single line on my clipboard. The channel's log file contains the right text, that is, with the two events each on their own line.

This was with 6.35, so whatever bug this is, it's still present. I have not seen it more than once, though.

Happened another 3 times now..