|Unix or UNIX?|
For years I used to use "UNIX" as I had seen in printed documents. Peter Salus's book  had me rethink and do a little bit of research, where he writes:
Doug McIlroy told me that he thought spelling UNIX with capital letters, rather than Unix, was a grave error.
I looked around, and noticed that people very close to Unix have been using "Unix" in a lot of places, and that Plan 9 documents mostly use Unix. I finally asked Dennis Ritchie about it. Here's his reply:
The difference came because the lawyers decided that all trademarks should be spelled in upper case, and UNIX was trademarked fairly early. They tended to force this in our published work. Since Unix isn't an acronym, just initial-cap, which I now prefer, seemed more logical. However, one complicating factor was that when we got a phototypesetter, we were so thrilled by it that we thought it was fun to spell UNIX in small caps just to prove we could do it. The UNIX or Unix trademark hasn't been ours for a long time now, of course. See http://www.opengroup.org/trademarks.htm and so we can do pretty much as we please (except in Bell Labs journals, which still care). Open Group, though, still prefers all-caps.
So I decided to use Unix since then, which is more friendly, less formal, and comes with a personality that a "trademark" lacks...