The case that arises quite often in practice is when a reference is retained, but can only be used on an error path (e.g., as part of an exception). The error c

The case that arises quite often in practice is when a reference is retained, but can only be used on an error path (e.g., as part of an exception). The error condition may not ever happen due to the way to code is written, but it's unlikely the garbage collector can prove that except in the simplest possible cases. Usually, it is acceptable to do a conservative approximation of the actual lifetime, and blame space leaks on the programmer. Garbage collectors usually also make assumptions in the other direction (a non-conservative approximation) when it comes to finalizable resources (e.g., file descriptors). These uses do not involve the reference that is visible to the collector. This gives us Reference.reachabilityFence(), and again mistakes are blamed on the programmer.