So - I updated some date-sensitive fixtures and re-ran all my rake tests... and suddenly everything went funny.
First off the rack: empty created_at date
The first problem seemed to be that the created_at date just didn't get set at all. I even printed it out in my test case with an "inspect" and it was showing: "created_at" => "0000-00-00 00:00:00". Pretty nasty.
I checked the fixture and even printed out the date I was trying to save (Time.now.utc) to make sure that Time was working in the fixtures... to no avail, until I realised I'd accidentally knocked off the .to_s(:db) from the end. I added that back and magickally my fixtures were loading the dates again (yay).
Next up: intermittent wierdness
So then there was an intermittant, but persistent error that was really weird. The method I was testing boils down to me pulling out a set of objects and ordering them by creation-date and checking the value of the last one in line. I was checking that if I created a new one - I actually got the new value out the other end. Now the problem was - sometimes I did... and sometimes I didn't... sometimes I'd run rake-test and it'd work... and I'd run it straight afterward and it'd break again.
Then I remembered that datetimes only have a one-second granularity... and some of my tests probably ran less than one second after setup. So I used 1.second.ago.utc.to_s(:db) in the fixtures and it all went away. :P