So I just tried to add the simple character word 瞭望 but it ends up as 了望. The problem is that 瞭 (liao third tone) can be interchanged with 了 because it is the simplified version of 瞭, but not in the word 瞭望 (liao fourth tone).
I actually had to look this up to make sure that I was right because there actually are some people who writes it as “了望”, but it is supposed to be 瞭望. If you would write 了望 at your hsk test etc you would not get a point for that.