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The single most useful thing you can do when filing a bug report is to include a numbered list of steps that trigger the bug for you every time, like this:
- Start the application
- Click on the File menu
- Choose Print
- ... etc ...
Not all bugs are consistently reproducible — but if your bug is reproducible, and can be reduced to a series of steps like this, it's as good as fixed once an engineer gets hold of it. Engineers love bug reports like this because it eliminates so much guesswork and trial-and-error.
Be meticulous and specific, but try to reduce the problem to the smallest number of steps you can. Say the problem occurs if you open an image, crop it, then save the image. Try eliminating the cropping step and see if the problem remains — it may not be necessary to trigger the bug.
Sometimes users file bugs against the designed behavior of the program: "When I do X and Y, Z happens!" — But, as far as the developer is concerned, Z is what is supposed to happen when you do X and Y.
To avoid this, it's best to include "expected" and "actual" results after your list of steps.