I honestly didn't see it, but thanks for that.
As for the altitude error. I was flying a short route of 250nm or so, and was heading up for FL350. When ATC instructed me to head direct to a certain point, it obviously cut out a chunk of my flight plan and my FMC decided that I could not make that altitude I guess? So instead I got an 'Unable CRZ ALT' or something along those lines.
So when I made the DIR To change, it made the crz alt error appear and I could not Execute the change or change the crz altitude mid flight.
I do remember getting one of those text boxes pop up as they often do - so perhaps it was the Exec button on the FMC that was actually unresponsive instead? Hard to know really.