We have a magento 1.7 site that's been up for ~3 weeks now. It handles ~100 or so purchases a day using Authorize.net and PayPal Express. About 1 in 50 customers get an error page saying the order cannot be completed (or something to that effect) even though the order is created and the payment is processed normally. It's happening with both PayPal and Authorize.net. Other than the user seeing the error page (and usually placing the order again thereby getting charged twice) everything about the order looks normal.
I've been all over the logs (magento and apache) and can't find any suspicious entries that correlate to the time the "erred" order was placed. Been trying to track this down for days now.
Any suggestions would be extremely appreciated.