Use a debugger. I've gone so far as to port my linux code to windows just to use visual studio's debugger, fix the problem, then port it back. Use version con

Use a debugger. I've gone so far as to port my linux code to windows just to use visual studio's debugger, fix the problem, then port it back. Use version control. If the order processing worked last week and johnny made a change to order processing in this release it's highly likely it's johnny's change. You make an assumption of where the problem is and start looking. Always understand your assumption can be wrong and you are looking in the wrong place. Avoid code with side effects. I've taken code with bugs in it and completely rewrote all of it without side effects just to avoid those types of bugs and magically all of the hard to find bugs disappeared.