Spy's myth-dispelling most welcome, but I wonder about subtle differences in wording and whether the post Transat checklist fixed it. The Transat checklist appeared to say "don't open crossfeed if a leak is suspected" which is subtly different from (quoting from memory) "you a required to confirm there is no leak before opening the crossfeed". Maybe this is the hole they fell into, in that they felt (if they had been checking fuel state at 30-minute intervals) they had no reason to suspect a leak.