It doesn't matter how long you've been a "developer" for, you always need to test things.
I say this coming off the back of spending this entire morning (yes, the same day as I am writing and publishing this article) going through this very site and fixing things with the theme.
I had recently been doing some updates to the parent theme, and without realising, inadvertently affected the site's child theme.
This essentially made my the responsiveness null and void. Developer error.
How long this was the case, I don't know, but it's only because some random person on Curious Cat sent me a "question" saying that my site wasn't very mobile friendly.
And I was shocked to find out they were right!
Even now I am still working on the theme.
Sure, I could have rolled back the themes or even just found old code, but in hindsight, it's better to re-write the code, to make it cleaner and up-to-date with modern technologies.
The main time-sink is the testing; going through each page and seeing what works and what doesn't on each of the browsers and devices; and there is even more testing afterwards.
So a huge thank you to the Anon user on Curious Cat for bringing this to my attention.
And it just goes to prove how you cannot just assume.
Some things are a given ("this should work first time", or "this works here, so it'll work here") and some things need to be put through the ringer, but everything should be tested and depending on how complex, all the more so.
The simpler, the better, IMO.
Update: A few hours after I wrote this, I broke the site. Luckily I had a backup of the working file that was causing the issue.
See, this is why testing and backups are important.