The reason many comedic movies work is because they reflect reality on some level, albeit a highly exaggerated one. Such is the case with the movie The Wedding Crashers. The reason so many brides and grooms relate to the movie is because deep down, they have the fear that there will be real wedding crashers at their wedding. Somehow, they also know that if their wedding is crashed by uninvited guests, those interlopers will not be as entertaining as Owen Wilson, Vince Vaughn, or the rest of the Wedding Crashers cast.
In fact, real life wedding crashers are much less likely to be strangers, than to be disgruntled exes and disenfranchised family members. Let's talk about the stranger danger wedding crashers first and get them out of the way.
For weddings held in public venues such as a hotel or large restaurant, it's possible that other guests or diners might wander into the reception area accidentally. They're the most harmless wedding crashers and will probably be good for a story or two at family gatherings for years to come. Under those same circumstances, you might find someone who is searching for free food and beverages, but other than that is of no real harm.
Much more insidious are thieves in search of gifts or cash. (Sadly, thievery is also a problem at some weddings at which no crashers are present - meaning invited guests or hired help are to blame.) Since some couples, especially those holding large receptions with hundreds of guests, can accumulate cash and presents in the multiple tens of thousands of dollars, attracting a criminal element is a distinct possibility.
Most couples set up the reception area with a table right near the door. The guests drop off envelopes into a basket or moneybox and leave the gifts right on the table. The table is often left unattended at various times throughout the event. A much better option to remove the temptation and opportunity would be for one person to be in charge of monitoring the table as the gifts and wedding cash envelopes come in and then locking them away in safe place once all the guests have arrived.
While it's unlikely that real life couples will suffer the fate of practically every soap opera couple ever seen on TV – namely the wedding being crashed by a presumed dead former spouse – much alive former spouses and other ostracized family members have been known to wreak havoc on weddings.
Celebrity weddings are known for their elaborate security, but any couple concerned about a possible security problem would be wise to consider hiring outside help. Wedding security personnel (yes, there are wedding security consultants as well) can wear uniforms or dress less conspicuously as a regular guest. Nobody but the bride and groom will know that the person is an undercover wedding security guard. Everyone else will assume the person is a guest of the other side of the family. You can also hire a real off-duty police officer in uniform as a visible deterrent to any kind of wedding crashing threat.
Another alternative if you are on the lookout for a particular wedding crasher (as opposed to a stranger) would be to assign the task of watching the door to a group of volunteers on a rotating basis. (You wouldn't want one guest or a handful of them to focus on that during the whole reception and not enjoy the festivities.)
Dealing with potential wedding crashers is like every other part of the wedding planning process. Plan for the best, but prepare for the worst.