I installed IWD's onepage checkout in Magento 1.7 a while ago and it seemed to work smoothly and integrate with my current template and other modules with no issues what so ever. However, a random customer had some issues regarding the checkout process and I received an email each time a certain error occurred:
Payment transaction failed. Reason SQLSTATE[HY000]: General error: 1 Can't create/write to file '/tmp/#sql_400_0.MYD' (Errcode: 17)
Checkout Type:onepage
I don't know how many times the customer tried to checkout that order but the number of emails I received are 8, each of them the same. A final time, the checkout process was successful.
The most unnerving thing about this is that I tried to replicate the error with a test checkout of my own and it didn't happen, leaving me with close to no idea what caused this, when it might happen again or how to fix it.