We know what Cullen did...
... to be sentenced, but we never find out the same about Joker, save for a hint that he's some kind of fraudster dreaming of a big score... Do you think there's any particular reason why Joker's crime was never revealed? I loved the film the way it was, but I'd also love to know exactly what Joker did, and I'm wondering if that information was held back, on purpose... ? What do YOU imagine he did?
"Your mother puts license plates in your underwear? How do you sit?!"