Why isn't "sed -n p" identical to "cat"?

[Michael Green]

Hello,

I am a little confused by section 6.1 in the manual. Especially the 
footnote #FOOT8. The manual states:

Section 6.1: When the end of the script is reached, unless the "-n" 
option is in use, the contents of pattern space are printed out to the 
output stream, adding back the trailing newline if it was removed[8].

Footnote 8:  Actually, if "sed" prints a line without the terminating 
newline, it will nevertheless print the missing newline as soon as more 
text is sent to the same output stream, which gives the “least expected 
surprise” even though it does not make commands like "sed -n p" exactly 
identical to "cat".

I tried executing "printf 'aaa\nbbb\nccc' | sed -n p" which correctly 
printed "ccc" with no trailing newlines. If there is a line that does 
not have any newlines to remove, it must be the last line, am I correct? 
How can "sed" send more text to the same stream if it doesn't have any 
more lines to process? Could you please give me an example demonstrating 
this behavior and the difference between "sed -n p" and "cat"?

Thank you for your time,

Michael G.