So, I bought my very first upright piano nearly 3 months ago after the numerous helpful replies I received to my questions here.
I've been playing it a bit now and then. Sometimes, while I was slowly sliding my fingers over the keys instead of entirely lifting them to move across the keyboard, I had this strange impression that I was "hitting something" inbetween some keys (like when you hit a speed bump while driving or something).
Today, out of curiosity, I crouched to look at the keyboard at key level, and I noticed at least one key that looked ever so slightly lower than the others. I didn't check, but I would say it can't be more than 1 millimetre, probably less.
I intend to ask the technician to check it on my first tuning, along with some other details.
In the mean time, I was wondering if it could possibly affect someone trying to learn piano? I don't think I have have the level to be affected by something like that on a noticeable scale yet, but part of me is thinking that your muscle memory will get used to the key being ever so slightly lower upon moving into another piano.