Summary and Info:

I wanted to install gnome-extra without tomboy. There is 31 packages in gnome-extra, so I entered them all but 28 (tomboy), hit enter, and I'm back to prompt, with '%' auto answered to [Y/n].

I tested a little further, and I found :
up to 62 characters, it works fine.
at 63 characters and up, it begins to behave strangely

if all numbers are valid, it seems to :
at 63, it leaves a cr or nl unread which auto validates the [Y/n] answer
at 64 and up, it answers '%' and validates, which is 'no'.

if there are invalid numbers, it's the same, but it answers "enter a selection", and it seems to feed it automatically characters above 63 or 64.

Steps to Reproduce:

# pacman -S gnome-extra
(enter one of those)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 (62 characters)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 2 4 (63 characters)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1 (64 characters)
222 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 2 (62 characters)
222 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 24 (63 characters)
222 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 2 4 (64 characters)

62 is ok
63 auto validates next input (usually a pacman question)
64 enters 'something', maybe the remaining characters, but not sure.