When making a new event in Calendar, you may experience a significant amount of difficulty navigating around the dialog. Buttons appear and disappear at random, elements are "showing alternate items" even though they're not, tabbing doesn't do much at all, and focus jumps to the top of the dialog with no warning.
Can not reproduce
on el cap, I open calendar, press cmd+n to create new event, then type in for example "gym on 6 oct at 11am", press enter. Then appears the dialog with fields already set in (subject, date, etc) and I can navigate it without issues.