Ok, here goes
Have to people in the same guild sit ready with 500/600k gold saved up, preferably side by side.
Wait for weekly reset to happen (I assume you can just emulate this on your test environment).
Have the first person start finishing the top 3 tasks (in any order).
Have the second person start finishing the bottom 3 tasks at the same time.
What happens to us, is that one (or both) of us will get the "someone else has contributed to this task" error a few times. Which should not happen, since we're the only two people contributing, and we don't overlap tasks.
Once this has happened and all gold has been spend, check the guild reward mail.
The rewards will differ between the two people, but they should be exactly the same.
I think this might have to do with two people finishing a task at the exact same time, but I'm not sure.
I do know it has happened to us two weeks in a row now, that we know of (never checked before).
Next week we'll try to alternate completing tasks (so not at the same time), to see if the glitch then doesn't happen.