Today I wound up typing and trying to use the following Vim command (more or less) with a straight face:

:.,$/Match.*Header.*Fields/d

Vim warned and prompted me by asking 'Backwards range given, OK to swap (y/n)?'. In an insufficiency of something (maybe coffee), I told it yes, and was then confused by the results.

What I had was a file that intermixed a bunch of copies of a header line (although each instance had a prefix that differed) and a bunch of data lines. I wanted to get rid of all but the first instance of the header line and keep all of the data lines, so I moved the cursor down a few lines below the first header line and typed the above.

What I was thinking was 'from here to the end' ('.,$'), 'match the header lines' (the /<regexp>/), 'delete the matching lines'. What had slipped out of my mind is that Vim doesn't match multiple lines when you use a bare regular expression this way; instead it goes to the first line that matches the regular expression. And instead of '.,$' and the regular expression being two separate steps, what I'd created was a compound single address, that went from '.' (current position) to '$/<regexp>/', the first line matching the regular expression (starting at the end of the file, which I believe defaults to rolling around to the start of the file). Since the first instance of the regular expression was at the start of the file, this meant that the range was backward, hence Vim's prompt.

What I actually was thinking of was the :g[lobal] command, and after I realized what I'd done wrong, that's what I used. I've used ':g' before, but this may have been the first time I've used it with a range (at least recently), and evidently my mind remembered the idea but forgot the important bit of the actual 'g'. So the real version I used, after I realized it, was only different by the addition of that one crucial character that made all of the difference:

:.,$g/Match.*Header.*Fields/d

(I'd blame my sam reflexes but even in sam this would have required a filter operation. It also would have been somewhat more annoying because sam isn't line based so I'd have had to make sure that the regexp covered the entire line or something of that order.)