What knocked Philip Seymour Hoffman off the wagon and led him down the path to his tragic death? One source says it was one "innocent" drink.

An insider tells TMZ that Hoffman, who'd been sober for 23 years before shooting The Master, gave in to temptation during a wrap party in 2012 and had a drink -- which led to a few more drinks. He allegedly admitted to friends that those drinks opened the floodgates.

A few months later, he began experimenting with heroin. Then last May, he checked into rehab because he was afraid he would escalate to IV use -- which he did. Hoffman was found dead with a needle in his arm last weekend.

Now the debate rages...what was the best PSH performance of his short career? Here are the Top 10: