I have written a simple server based on this echo server and observed the following behavior:
when the connection is closed, the read operation gets 0 bytes read and the buffer id 0, and then "returns" it to the available buffers using a "provide" call.
In reality no buffer was given to that read and it is returning buffer 0 without ever getting it, which results in any operation that uses buffer 0 to have unexpected results due to multiple operations sharing the buffer.
Removing line 153 fixed this.
Might be important: my version uses UNIX sockets and not the INET ones used here.