I'll share a key transpose horror story that resulted in me swearing I would learn to play in every key and never, ever again use a transpose.
My main axe is the drum kit but I'm a pretty decent keyboard player (now). However, back in about 1991 I had just enough keyboard chops that I was able to join one of the hottest 60's-80's cover bands in town (backing several national acts, playing large festivals, etc). I did not think I was good enough, but keyboard players are alway in demand - and I figured what better way to really improve my chops, playing in a good band with very good players.
So I woodshedded all the tunes and worked harder than the other guys each time we had to learn a new tune (I often had to cover horn lines, strings, etc. in addition to the standard piano & organ lines) - I had two keyboards and two modules, so I was earning my pay. But I was also getting better as a player in leaps and bounds.
Well, maybe a couple of months into the gig a couple of horn players from another well known group sat in and we were going to do a few R & B tunes. The first tune they called off was originally recorded in Bb (horn songs are often in flat keys) - however, I had learned the song in C and I was not yet good enough to transpose on the fly - so, I used the transpose button.
I got through the 1st song OK - but then - at the start of the next song I forgot to adjust the transpose, naturally I started the next song 1/2 step flat (and even a deaf person could have heard the chaos). I had to drop out of the song while trying to get into the correct key (occasionally hitting a note here and there as quietly as possible to figure out what key I needed to be in. For some reason, I could not recall that I had transposed down 1/2 step - so I was not able to logically simply transpose up 1/2 step. I pretty much fell into a panic mode, all rathional thought failed me and I no doubt had that "deer in the headlights look".
The guys in the band kept looking at me as if I were a troll and the sound guy finally took me out of the mains. Every single person in the crowd (a couple of thousand people) could see (and hear) that I was having a problem - and if I did not believe that "the show must go on" - I would have crawled under that stage into a fetal position until the next morning.
I never, ever used a transpose button after that and the next day I learned that friggin tune (whatever tune it was) in Bb!!!
Bottem line - learn how to play in every key!!