Running Linux Mint (based on Ubuntu, based on Debian GNU/Linux) means this system doesn't come with Arial 'built-in'. Hooray. Except that for a while, some web pages in Opera in particular have displayed oddly here: some of the sans-serif fonts have been very thin and narrow.

I did wonder if this was the result of having Helvetica Narrow somewhere on my system and, yes, it looks like that's what happened. Although it wasn't in the directories mentioned in /etc/fonts/font.conf as the places to look – the three main ones are /usr/share/fonts, /usr/local/share/fonts and ~/.fonts – it looks like having run FontMatrix at one point in the past created ~/.Fontmatrix which had links to Helvetica Narrow amongst some others.

Deleting that directory, running sudo fc-cache -f -v, restarting Opera and all's well. I don't know what was getting its fonts from there, but at some point I will see what the CSS for the affected sites was asking for. It probably wasn't Helvetica Narrow…