What errors that can't be detected by testing techniques but rather by guessing?

Error guessing is known to be one way of knowing the error from previous experiences. however, it's a technique that usually testers perform if all other formal techniques (like Whitebox testing or other blackbox testing) failed to identify the error.

Scenario: Assuming that I have automatic bus door that opens once the bus arrives at the station for people with disabilities.

My question is as follows: What kind of errors can take place (in the scenario above) and can be detected only by guessing not any other formal techniques?

Thanks..