The story

Maybe the story began when I noticed that a status message (a line displaying text at the bottom of the window) looked somewhat unusual. Or maybe when I checked the log file to investigate, and found out that the program reported “______ file updated”. That looked as a typo, or unsupported characters in the filename.

Inconsistencies in the log file were considered a minor defect on the project. I wasn’t even sure I should mention that.
But then I took an older log file, and discovered that report looked differently: filename wasn’t missing; it was readable.

Next step was to check the timestamp when the file was modified – and the timestamp didn’t match recent processing operation.

Having my suspicion confirmed, I went to the program, made changes that must affect the file, and started processing again.
Voila! “______ file updated” but the file is not updated.

The lesson

That was a real bug, and pretty much serious. Discovering it on purpose was merely impossible. However, using a minor defect as an indication of a problem area combined with en extra effort taken, helped revealing a bug.
Walking extra mile in testing often pays off well!