BufferLedger has a HistoricalLog which records stack traces at various events in the buffer's life, such as when it was first allocated and upon incrementing/decrementing reference counts. While somewhat of an implementation detail, this information is useful for debugging, especially for tracking down memory leaks. We could document it (with a suitable warning).
However, the stack trace is less useful than it first appears because it accidentally truncates the log. We should fix that too.