Final answer:
The most likely complication of intracoronal bleaching with an inadequate seal is external root resorption, due to chemicals leaking and stimulating the periodontal ligament.
Step-by-step explanation:
The most probable postoperative complication of intracoronal bleaching for a tooth that has an inadequate internal seal is external root resorption. Intracoronal bleaching involves the use of chemicals, often peroxides, to whiten the discolored tooth from the inside out. However, if an adequate seal is not present, these chemicals can leak out and lead to the stimulation of the periodontal ligament, potentially causing external root resorption.