There seem to be two schools of thought for getting to the bottom of a piece of buggy software: those who log everything vs. those that use the debugger. I've been surprised to learn how large the former camp is, at least among those developing for web-based applications. I'm not saying that one should use one approach exclusively, just that the debugger is underused in web programming.
I suppose a reason for this is the perceived difficulty in debugging server-side code from a client machine, but this is more a case of lack of knowledge than an actual technical issue. Most application servers support some facility for remote debugging. Similarly, in Java, at least, the virtual machine itself can be configured to accept connections from remote debugger clients (if, for example, you wanted to configure the JVM to listen for debugger connections on port 8000, you'd include this in the command-line arguments: -Xdebug -Xnoagent -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=8000). Once started in this way, the debugger can connect to the remote JVM and do everything it could do if it were running on the same machine.... you can set breakpoints, inspect the contents of memory, evaluate statements and see the full stack trace for all the running threads... all of which are quite powerful when looking for an elusive bug.
The heuristic I use for deciding whether to use the debugger or a logger to find the root of an issue is as follows:
If the bug occurs infrequently and it is not clear how to duplicate it, I use a logger to see if I can detect a pattern. If I can, then I'll use the debugger to look deeper into things. Conversely, if it's clear how to duplicate the bug, I'll usually jump right in with the debugger.
Here are some resources for remote debugging:
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