On at least some Macs well into the 2000s, I remember that if you had a dialog with a text field in it, Return would add a new line in the text field, but Enter would _always_ choose the button with the thick black bar around it (typically "OK"). There were also some websites where OmniWeb (remember OmniWeb?) would interpret the Enter key (but not Return) as "click the 'next' link on this page", which was great for paging through webcomic archives and the like.