Luma was doing that
A redesigned product page came back without a wishlist, which went on a growing list of things Magento's default theme had been doing for years without my ever asking it to.
Somebody else's checkout
Next week Shopify stops supporting checkout.liquid on 3 of the checkout steps, and going through one store's customizations turned up 8 pieces of decoration and 1 thing that was the entire business.
Shopify collection limits
A row of six related products came out one card short on about a third of a client's catalog, and the thing eating the missing card was a limit I'd never read about, being spent by a block I hadn't written.
Can the client do that themselves?
An account manager watched the Page Builder demo in April and asked whether the client could do that themselves, and my job since then has mostly been deciding what that means.
Cron schedules and cache windows
The merchandiser at a client could tell me how long a price change took to appear on the store, morning and afternoon separately, and nobody on our side could have told you either number.
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.
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.