I wss doing something in Excel and needed to aggregate some data. I figured there must be some aggregation function. I looked it up and, sure enough, I found DSUM() and DCOUNT().

I spent a few hours finishing my analysis, and then went to someone in the finance team to show them what I had discovered.

They noticed I had many many similar-looking aggregations in a grid, and asked "Why not just use a pivot table?".

My reply: "What's a pivot table?".

Also in the same job I learned the hard way why you should never use OFFSET() if INDEX() will do the job.