I have both showed up to the wrong airport as well as on the wrong day (or conversely having booked the wrong day without noticing it) and I found that a healthy dose of self-deprecation in those moments is the only path to a positive resolution. Same goes for the handful of times in my life I have woken up in a hotel room several hours past my ETD.
I have booked a number of flights over the years ...in my head but forgot to inform UA of the arrangements..........never felt they were accountable for that ,seems like a situation for grovelling not yelling at someone