In a word, no. Formal events tend to be pre-planned affairs, and your uninvited guest may find themselves without a chair or a meal all night. They, and you, may also be the recipient of icy glares because of your party-crashing ways.
As with all good rules, there are exceptions. If you have a significantly significant other that the host has forgotten or isn't aware of, you can ask the host if you can bring a plus one — but only if you're on familiar terms with the host and it won't be a great inconvenience to them. In other words, contact the host in good time and be polite about it. A breezy text message the night before the event might get you and your date uninvited.
