Crashing a wedding is possible depending on a number of factors. Why you would want to crash a wedding is a whole other thing, however crash success is largely dependent on how many people are attending, whether the bride and groom notice or care, and how intoxicated everyone is by the time you arrive. Some wedding crashers walk up to the bride and groom, congratulate them, and hand them money, while others use the crash as an excuse to take advantage of the open bar. Crashing a private family affair is in very poor taste, however if the wedding is a huge event and no one is going to notice you anyway, you can probably get away with it.
Again, think about your reasons for wanting to crash a wedding. Is it the wedding of your ex-girlfriend or boyfriend? Former friend? Rich socialite you don’t actually know? Relative you don’t get along with? Consider your reasons for more than a few minutes, as you could end up causing a scene…and seriously embarrassing yourself.