Create templates to quickly answer FAQs or store snippets for re-use.
Maxing out the available disk space on a server, crashing the database, and bringing down our web app. It was a catch-22, I couldn't reduce the database size. It caused a day of downtime.
Or the rookie dev move where I expanded all the objects everytime a page was requested. The tree was some seven layers deep, and made a request for each object one at a time. Uh, there were better ways to approach it. lol, the dev server nearby our desk would literally whine (power up) when that happened.
But these experiences do help you gain empathy for that next dev who makes a blunder.
Pushed a job queue in production that sent a 1-time email to customers greeting them on the next day after they made an account.
The job-pushing code was configured to run once per hour, picking new accounts from the DB and sending the emails.
I mismatched transactional/non-transactional, and we got emails going about 24 times a day for most customers, and lots of complaints.
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