My biggest dev epiphany was also my dumbest one. We were working on a payment system for a roadside rescue company where an employee would register payments "in the field".

The challenge was automating input with typeahead and autocompletes in order to lessen the workload as manual input had to be an absolute minimum; this will be used by truck drivers/mechanics as they are trying to hurry to the next customer who has been waiting for 3 hours longer than we said we'd take.

We managed to make the invoice path first (customer has not paid, employee logs personalia needed for billing), but when it came to "paid on site" we almost upended the entire system trying to find a way to fetch user personalia outside of the invoice path.

Neither of us realized it during the days we were banging our heads against it. Realizing we don't need to make an invoice for a job that has been paid for was equal parts relief and utter embarrassment.

Probably my greatest lesson in how important it is to pull my head out of the code once in a while, and to ask myself what I'm trying to do and why.