One of testing scenarios which made invalid item-not-found error in mod_http_bind (and i fixed in b34dafa7ac32f49d806fea0c8286d4c492130bfd), generate similar results in mod_bosh.
Here is how it can be triggered
1) Message with rid=A arrives
2) Server send response to 1)
3) Message with rid=A+2 arrives, server delays processing of it until message with rid=A+1 is received
4) Another client sends message which needs to be delivered
5) Server immediately send message from 4) as response to request from 3)
6) Message with rid=A+3 arrives
7) Server send item-not-found error because rid is too big
Fixing this will require to change in step 5, that message delivery would need to wait until message with rid=A+1 finally arrives.