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https://www.istqb.org/downloads/category/2-foundation-level-documents.html /wiki/spaces/CUST/pages/1899496760
BUG/Defect - everything what is not working as it should 
(based on requirement, general knowledge of the system etc.)
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Often used techniques by BBT coming from Boundary value analysis, State transition testing or decision tabels (please reffer refer to https://www.istqb.org/downloads/category/2-foundation-level-documents.html or any other source to get familiar with this techniques)
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