Once upon a time in a Fab far away...<p>We had received three reports over as many months of a system that failed mysteriously in a manufacturing setting. We sent a field tech out each time. He examined the system, examined logs, found that an error had happened, but found no explanation for it.<p>On the fourth report, while examining the system, the field tech noticed that single thread of sunlight was passing through a high window and into the viewport of the system. He immediately realized that the optical sensor on the automated arm would be spoiled by that beam had the system been operating.<p>We had the manufacturer cover the window, and filed it as a bug that only happens at a specific time of day in a specific season when the sun is shining.