Been there, done that!
And I'll bet that you'll never have that happen again having realized what a pain it is when that occurs. For the rest of your career, either you (or your assistant) will be quick with the spatula and mixing pad and getting that freshly loaded new crown or cast post fully seated well within the working time of the cement! Plus, this is one of the main reasons why I keep a small timer in my operatories. There is no guessing then about how much time you have left to work with a cement or has the full set time been reached on a cement or impression material or temporary material, etc