Here's a daft thing: a lot of browsers, typically on phones and Macs, won't re-download a file if it's been downloaded before. I can understand caching pages, images and CSS, that's good, but caching downloaded files? Meaning that when a user clicks to download a Word doc or a PDF, the browser will decide that they don't need to! Even though they think they do! I'm now having to add ?v=time() to PDFs, Excel files and similar, which feels really hacky. Some browsers will ask if the user wants to re-download, which is fine, but taking people to old and obsolete versions of documents when they want the current version is just stoooooopid.

Very early on I was putting together my first database-driven web app and decided to use record IDs with leading zeroes - you know, because that would look neater. It was a long, long time before I came to know that a number with a leading zero would be evaluated as octal, which in turn meant that any number with an 8 or a 9 in it was invalid, which then meant it would be a match for any other invalid number and retrieve a seemingly random record.