Coin Metrics estimates that nearly 1.7 million bitcoins are lost through repeated transactions, rewards unclaimed or stolen. The report classifies lost bitcoins into two types of calculations: to prove that lost bitcoins are lost and that may be lost. It can be proved that the lost bitcoin means that it can be proved permanently lost, and that possible loss represents a great probability of permanent loss.
Bitcoins that can be proven to be lost include Genesis (originally produced Bitcoin), duplicate transactions, unclaimed mining rewards and OP_return output (script code for hosting additional transaction information).
Before OP_RETURN output was standardized, a large number of bitcoins were sent to fake addresses. The report says there are currently 2,200 BTCs in three fake addresses that will not be lost forever but are unlikely to be recovered.