Plaintiff did so here, agreeing with defendant (through counsel) to extend the time to file a motion for attorney's fees on appeal while the parties tried to resolve the matter informally. Sure, there's a rule that says that you have to file a stipulation to that effect, but you can trust the promise of your brothers and sisters in the Bar, right?
The parties can't agree, plaintiff files a motion for fees, and the defendant objects on timeliness grounds. Defendant's lawyer doesn't deny granting the extension, but argues that since it wasn't filed, it doesn't matter. The trial court agrees.
The Court of Appeal reverses. That's a legitimate basis for a CCP 473 motion. "Excusable neglect" includes relying on the promise of the other side's lawyer. At least here.
Good to know. Plus, on the merits, I agree.
Still, let me give you this advice. File the stipulation anyway. It's lots easier than going through CCP 473 motions. Lots.