I get that. My point is: how do you "accidentally" take 6 different medications at once?
He probably took them over a period of a couple of days. They were prescription medications, so he was (at one time or another) told he should be taking them by his doctor(s), but probably not all together. When you're not sleeping, exhausted, sick and jetlagged (as Ledger evidently was), you make mistakes, lose track of time, what you've taken and when you've taken it, and make errors of judgement about what you should be taking at all.
It's quite possible that he was taking "safe" dosages (there was no indication in the autopsy that he had excess of these drugs singly in his system), but all together they killed him, especially if he was sick with pneumonia.
There is actually a medical term for drugs that are safe when taken separately, but when taken together cause harm, I think it's "drug synergy". There are so many things that can affect how a drug works and metabolises inside a person, but other drugs can be one of the big affecting factors.
For instance, there are two main types of sleeping pills (IIRC sedatives and hypnotics). Those to get you to go to sleep, and those to keep you asleep. You're not really supposed to take both together, and they can have a half-life in the body of over 100 hours (eg diazipam). Side effects could make your forgetful so you take too many, and wake up feeling just as bad as if you hadn't slept at all, so you take more of them, etc. If you're taking them together, you've got double the dosage working on the same mechanisms of your body, then you're taking more of them because you're not being careful enough to track your usage, then they are leaving you feeling exhausted and irritated as if you haven't slept at all regardless of the medications you took.
A person gets off-kilter and screws up. Takes too many pills in the space of a few days, mixes medications they shouldn't mix, gets pneumonia that makes them weaker than they normally would be, and one day never wakes up.