I don't know how you could "accidentally" OD on anything either besides forgetting you took some and then some more but even then, you would most likely need to be taking quite a bit of medication. There are a couple of ways overdoses can be "accidental", pharmacology isn't my field, so these will only be the simple ones. 1. Drug interaction or synergistic effects. Some drugs basically fuel eachother's fire, so to speak and the total effect is greater than the sum of it's parts. For example, you can take a therapeutic dose of drugs A and B and die from it, even though you took a safe dose of each one. This is often illustrated with alcohol and pills. A user can take an amount that would be within a safe range, and drink an amount that would not be toxic, but they die because of the combined effect, which is toxic. 2. Opponent processes. The environment in which drugs are taken comes to elicit a response which essentially moves in the opposite direction of the effect of the drug. So if you took a strong sedative in your bedroom, that room and the range of stimuli present would elicit an excitatory response over time. Go take the same pills in a novel environment, which does not elicit this response, and the effect of the drugs will be stronger. It's often discussed that heroin addicts use a familiar amount in hotel rooms and then die, while that dose was administered countless times at home. Ever feel drunker than you should be when you drink in a strange situation, like the middle of the afternoon with new friends? This is probably why.