Everyone was Susan
A full-page cache took the store's category pages from about 2 seconds to 300 ms and broke four things, and all four turned out to be one piece of the header being rendered in the wrong place.
content: "\00a0"
Why the most-used string in my stylesheets is an invisible one, how CSS escapes will silently eat the character after them, and the parts of the content property that aren't strings at all.
Nobody prints anything
Except invoices, tickets, recipes, directions, order confirmations, and every page a client's accountant touches, and the stylesheet for all of that is about 40 lines nobody thinks to write.
How template fallback picks a file
A 10-minute ticket ate most of a day because there were five copies of the template on disk, I kept picking the wrong ones, and the right one didn't have the dollar sign in it.
It wasn't just my machine
Getting the local environment running on my Windows machine took most of my first week and hours of the lead developer's time, and I spent all of it assuming the problem was me.
See email from Casey
The ugliest 40 lines on a practice's website sit under a comment telling me not to change them, and the email that comment points at stopped existing years ago.
word-break and overflow-wrap
Two properties that sound like synonyms, do different things, and get reached for in the wrong order roughly every time a long URL breaks out of a card.
What the CMS does to your hero image
I got a homepage's hero down to 180 KB in January, it's 2.4 MB now, and the only thing anybody did was change the picture through the form that exists for changing the picture.
git bisect
I bisected a broken stylesheet by hand last week, through a pile of backup copies on a live server, and it made a better case for version control than anything I've said out loud.
Alt text is writing
Every image on the site had an alt attribute, the checker reported zero errors, and not one of those attributes told anybody anything because filling in a field and writing a sentence turn out to be different jobs.
Worth doing badly
Chesterton's line about doing things badly is the most useful thing I know about actually finishing anything, and it goes well past side projects.
optgroup
A select with 40 options in it is nearly always a select that wanted three or four labeled groups, and the element for that has been sitting in HTML the entire time.