Risk are events that has a chance of occurance and a potential loss if it occurs.
There are several types of risk and testers have to take these into account when creating test plans.
Some risks are
budget risks (due to estimation or cost),
operational risks (insufficient resources, etc),
technical risks (new technology, unknown technology, emerging trends, etc)
Lack of understanding of requirements
Change of requirments
Lack of design documents
Testers have to really take into account these risks when planning for testing.
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