Totally bizarre error, requesting assistance

[Koz Ross via Digitalmars-d]

I don't know if this is the right place to ask, but I figured it 
was worth a try. Basically, I'm getting totally bizarre behaviour 
from my code, which I have put in a repo at 
https://gitorious.org/tournament-tree/tournament-tree/source/545460fb9d760b6bfa5bc74deb7b69d79ec70541:

To replicate the error, simply run 'make profile' from the 
project directory. It should fail (claiming an enforcement error 
on node.d, line 50). Then, go to tournament.d, and uncomment line 
33. Then, try 'make profile' again, and it won't have the same 
issue. I've been working all day to try and isolate the source of 
the problem, across multiple machines, and I've had no luck. This 
is the most isolation I've managed.

I would really appreciate any and all help, as this is something 
that totally baffles me. I know this is a lot of code, and it's 
probably quite hideous, but it would help me a lot if someone 
could at least suggest as to where the source of the problem lies.

I already am aware that the failure is caused by an overflow, but 
its source is completely unclear, and I never get overflows when 
that one writeln statement in tournament.d is executed (hence my 
confusion).

I'm running Manjaro GNU/Linux, and my gdc version is 4.9.1. I 
have a 64-bit OS.