deleting a req associated with an exchange crashes website

[cgorringe]

occurred on 11/10/2012 starting at 8:06pm
more info forthcoming...

[cgorringe]

see this for fix:
oscurrency@47c3329

in the absence of this fix, when the problem occurs, you find the exchange with the missing req. create a new req with active=false and then associate the req with the problem exchange.