Worst wedding I've ever been to was one I was helping my ex photograph (he's now an award winning wedding photographer but this was an early venture at this stage). He'd been asked to shoot at a colleagues wedding.
First of all we knew no one - not uncommon in wedding photography really. But it was that weird juxtaposition where he'd just started out doing wedding photos but was still seen to be part of the party, and the boundaries weren't really clearly defined as we were super young and didn't know how or what, it was total inexperience on our part and was a casual favour for the wedding party more than anything!
The wedding itself was fine, but once everyone descended to the reception things started to rapidly decline. Food was bloody awful, but loads of money was behind the bar so people were twatted before food even arrived.
The best man speech went on for 45 mins except the poor man clearly was not well versed in public speaking and was so abjectly terrible at it that it made it seem a lifetime. Think awkward pauses, zero social skills, no jokes, just a recital in a monotone voice for 45 fucking minutes. He was completely wasted which didn't help.
We were committed to staying until the first dance, except both the bride and groom were pissed and could barely stand. Still, we stayed, but left shortly after it was apparent the first dance physically couldn't happen as they were both asleep on a sofa in the foyer 
As we were leaving, the hugest fight broke out between the opposing families which resulted in the police being called and FIVE people from the wedding party taken away in handcuffs. As tempting as it was, we didn't take photos of that particular bit 