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What is The most common consequence of bleaching nonvital teeth is: and explain in detail?
There have been reports of external cervical root resorption following intracoronal bleaching procedures.
This is probably caused due to the diffusion of hydrogen peroxide through unprotected dentinal tubules leading to resorptive changes in the periodontium. 
The caustic nature of hydrogen peroxide can cause the necrosis of the cementum and the periodontium, which in turn results in initiation of inflammation and subsequent resorption. 
Resorptive process may be worsened with the application of heat for bleaching agent activation. Hence, application of an appropriate intracoronal barrier is mandatory for intracoronal bleaching procedures.