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Thomas's answer probably has to do with the risks of decrypting a stream and being unable to authenticate it first. (See also: the Cryptographic Doom Principle.)

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But it is still safe to do random access AES-CTR if you've already checked the MAC. Random access is not a problem, decrypting before authenticating is a problem.

I think that's why the statement was "be careful with" rather than "don't"

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