If an error that you are trying to exclude is one of those unpleasant errors that appear only on certain computers or that is difficult to reproduce, look at it from different points of view. This is the step in which you should start thinking about the inconsistency of the versions, the differences in operating systems, the problems of binary files or their installation, and other external factors.
Surprisingly enough, the method which sometimes works is to forget about the problem for a day or two. Every once in a while you are so focused on the problem that you do not see the forest through the trees and start to miss the obvious. Dis-focusing from the error, you give a chance the subconscious to work on the problem. It is clear that everyone reading books on software testing / debugging encountered errors on the way from work to home. Of course, it is difficult to dis-focus from the error if it delays the delivery and the boss breathes down your neck.
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In several of the companies a certain tester worked in, the interrupt of the highest priority in the software priority based scheme was something called “conversation about an error”. This means that you are completely knocked out and should discuss the error with someone in detail. The idea is this: you go to someone’s office and write your problem on the board. So many times the person came to someone else’s office, opened a marker, touched the board and solved the problem without saying a word! It is the preparation of the mind to present the problem that helps to pass by the tree you have bumped into, and to see the forest at last. A person you need to discuss an error with should not be chosen from among the colleagues with whom you work closely together on the same part of the project. Thus, you can be sure that your interlocutor will not make the same assumptions about the problem as you do.
Interestingly, this “someone” does not even have to be a human being. By the way, cats turn out to be great debuggers, and they have helped their owners find a lot of nasty errors. One man collected these together, outlined the problem on the board and let their supernatural abilities work. Of course, it was difficult to explain what is happening to the postman standing at the threshold, given that on such days the man did not take a shower and walked inside in his underpants.