GC pathological case behaviour

[John Colvin via Digitalmars-d]

On my machine (OS X), this program eats up memory with no end in 
sight

import std.concurrency;
import core.stdc.stdio;

void start()
{
     while(true)
     {
         receive(
             (int msg)
             {
                 char[2] s = '\0';
                 s[0] = cast(char)msg;
                 puts(s.ptr);    // remove this => no memory leak
             }
         );
     }
}


void main()
{
     auto hw_tid = spawn(&start);

     while(true)
     {
         send(hw_tid, 64);
         auto b = new ubyte[](1_000); // 10_000 => no memory leak
     }
}

This is very odd, no? I'm not sure if it's a bug, but it sure is 
surprising. Why should "puts" cause a GC leak (writeln is the 
same)? What's so special about small allocations that allows all 
my memory to get filled up?