Is it a good practice to put bug numbers in the file itself inside a header comment? 

The comments would look something like this:

 MODIFIED    (MM/DD/YY)
 abc 01/21/14 - Bug 17452317 - npe in drill across in dashboard edit mode

 cde 01/17/14 - Bug 2314558  - some other error description


It seems helpful, but is it considered bad practice?

I've seen this done before, both manually by authors and automatically by scripts and triggers integrated with version control systems to add author, check-in comment, and date information to the file.

I think both methods are pretty terrible for two primary reasons. First, it adds clutter and noise to the file, especially as these comments age and become irrelevant to the current state of the file. Second, it's duplicate information from what's already maintained in the version control system, and if you are using a modern version control system that supports change-sets, then it's actually losing information about changes.

If anything, consider integration with your defect tracking system. Some tools allow you to link a defect or task ID number in a check-in comment to an item in the tracking tool. If you have all of your defects, enhancement requests, and work tasks in the tool, you can provide linkage that way. Of course, this comes with the downside of a dependency on those tools for the project.
