Translation of summary of last part, "Techniques, used for debugging of Production applications" just published. This is almost end, only appendix about backup techniques and epilogue left.
In the last chapter we discussed methods of testing problems which can happen only on production server. Lets repeat them:
Method #25: if something unexpected happens check error log first.
Method #26: turn InnoDB Monitor to on to have information about all InnoDB transactions in the error log file.
Method #27: use slow query log to find all slow queries.
Method #28: use MySQL Sandbox for fast and convenient testing of your application using several versions of MySQL server.
Method #29: use part of data when work with queries which return wrong results from huge tables.
Original text is here