Editing Module:Buffer/doc (section)

=====Buffer.last_concat=====
When strung without a ([[#valid|valid]]) <code>sep</code>, the result is cached at <code>Buffer.last_concat</code>. Until purged, future calls to return that Buffer as a string will return this index instead.<ref group="note">Using {{luaself|:_var|plain=y}} prevents future caching on all Buffers, though Buffers which already unmodified Buffers will continue to return their cached version</ref>

This should clear automatically whenever a Buffer object function changes the contents of a Buffer. You may manually purge the cache by setting this key to nil, as well as by passing nothing to {{luaself|:_cc|args=()}}.