I am using Version 1.50 and had the following problem just once.

Customer filled basket, went to checkout, payment was authorized and transaction ID was generated by authorize.net, however order was not created. Unfortunately the shop owner didn't contact me on time for me to check the server logs to see if autorize.net had a problem when connecting with our server, thus I cannot recreate what happened.
In the database I can see the full cart and follow the customer's actions until the autorize.net took over.

Any ideas what might have happened?

All other orders had no problems.

Thanks in advance.