It's being reported in the mainstream media that the casualty count in the Aurora shooting could have been much higher but for the fact that the shooter's semi-auto rifle jammed. The Washington Post says authorities found a 100 round capacity drum magazine in the theater. While it doesn't say whether the magazine was empty or how many unexpended rounds it contained, if it was not empty, it could well explain the malfunctioning of the rifle.
Most of those familiar with military firearms know that drum magazines, while capable of providing a large ammunition capacity, have a long history of being prone to jamming more frequently than the more commonly used 20 or 30 round, straight-feed, box magazines. In fact, one of the main complaints about one of the most widely used weapons of WWII, the Russian PPSh-41 submachine gun, was the frequency with which its 71 round, drum magazine jammed during firefights. American troops had similar complaints about the less widely used Thompson submachine gun when equipped with a drum magazine.
It is likely that the Aurora shooter was unfamiliar with this reality and was seeking maximum shooting capacity without having to reload, thus his selection of the 100 round, drum magazine. Fortunately for theater patrons who escaped being shot, the drum likely performed in accordance with its design's reputation for unreliability.
This magazine issue might have some legal bearing on the prosecution of the case if it can be shown he bought the drum magazine separate from the weapon, which could be construed to indicate he took additional, deliberate steps to achieve maximum killing capability.