At my church gig, I regularly play hymns with the music director/organist. This particular Sunday we had strings playing with us as well. A last-minute discovery of key differences (strings had hymn in D, hymnals were in Eb) meant I had to play the hymn a half step different than written. Which is fine - I've become so familiar with the repertoire that changing keys is a no-brainer (similar to transposing happy birthday). However, it helps when you remember the correct direction the key is supposed to be moved.
After the intro, I entering strongly in the key of E. I was immediately disconcerted by how bad the music sounded against the accompanying players. I immediately wondered if something was wrong with the other players as the pastors glance at me oddly . . NO, I realized a nanosecond later: wrong key - wrong key - change key now!!
Churches generally put a positive spin on things - which means I heard several versions of "you have a real gift for transposition" after the service. I am fully aware that the phrase really translates to, "wow, that sounded really bad before you changed keys", so it takes some will power to accept the comment gracefully.
Events like this are examples of why pianists should transpose well. And why they should write things down.