Durability (the D in ACID) means that if the power goes out or somebody kill -9s it, the database will be in a consistent state and anything that was committed was in fact committed. In current and previous versions of PostGres, pulling the plug at the wrong time could leave hash indexes in an inconsistent state. That's fixable by deleting and recreating the index but the time required to do so is proportional to the table size. In the next version of PostGres, it won't be a problem and hash indexes can be used.
In the case of a primary key, a row is inserted or deleted but the power goes out before the hash index is updated. When the power comes back, the hash index still points to rows were deleted and is missing rows that were inserted.
In the case of a primary key, a row is inserted or deleted but the power goes out before the hash index is updated. When the power comes back, the hash index still points to rows were deleted and is missing rows that were inserted.