Error page when placing order, otherwise order is normal

Question

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.

Answer 1

This error can arise in a number of ways. If there is a DB error when saving the order (sometimes a deadlock or lock wait timeout due to a running index) this error will return. You can try

show engine innodb status

in MySQL to see check the most recent deadlock. The PayPal module throws this error when it encounters problems with the response data returned from PayPal.

In either case, if you have exception logging on (System > Configuration > Developer > Log Settings), you should see the related exception in the exception log file.