One of the interesting thing that troff lled to in Unix was stderr!   Initially, Unix had stdin and stdout, but no stderr.

We had a phototypesetter that worked by flashing pictures of the various letters on photosensitive paper.  It came out in a roll that had to be fed into a developer and when it emerged it had to be cut into pages manually.   Several months after we got the phototypesetter somebody (The Hunt brothers?) cornered the market for silver, and the cost of the developer shot through the roof.   We were asked to cut back on our use of the phototypesetter, and we tried to do so.

One day, I was attempting to print out a one-page document.  I ran troff piped into the phototypesetter and got out my roll of paper and fed it into the developer.  Out the other end came a page with only one line on it (beautifully typeset): "cannot open input file xxx"   A number of others had similar experiences, and stderr was born...