Ledger's mistake was... mixing.

According to the AP/CNN, Ledger had a shitload of drugs on his system. Talk about a Molotov Cocktail:

"Hydrocodone and oxycodone are painkillers. Diazepam is an anti-anxiety drug commonly sold under the brand name Valium; alprazolam is also an anti-anxiety drug, sold under such names as Xanax. Temazepam, sold under such names as Restoril and Euhypnos, is a sleeping agent, as is doxylamine."

Six drugs in one body. One little girl without a father. Damn, people. Damn.