Inverse stopped clock

Genius is no guarantee against being dead wrong. A gris-gris is a voodoo amulet, something you carry or wear to bring good luck and ward off bad vibes. We have a saying around Bullshit: Everybody got a gris-gris. What we mean by that is everyone believes in some crazy thing they just can't quite give up yet. An inverse stopped clock refers to a situation in which someone who is usually logical, rational, or correct does or believes something idiotic or crazy. Also referred to as “A running clock is only guaranteed to be correct that time is passing”, "Sometimes even geniuses make mistakes", "Even Homer sometimes nods", or the common phrase "Nobody's perfect."

Explanation
Due to the imperfect nature of human beings, even reasonable people &mdash; including people widely respected for being reasonable &mdash; will make a mistake now and then, often by chance alone. Perhaps something cranky (e.g. a conspiracy theory) appeals to someone's ideology and they go with it, even if they're usually correct on other subjects. Other times, they may panic and do something stupid in response to a sometimes exaggerated threat or situation. The difference between these people and other cranks is that they are usually not crazy, but every time an inverse stopped clock situation takes place, their reputation becomes slightly more soiled. (The degree of stupidity also makes a difference, and if the moment is crazy enough, they may never be able to live it down, with everything they say up for question after the incident.)

Dismissing everything a reasonable person has accomplished or believes in simply because of one or two crank beliefs, silly superstitions, or terrible behaviors can be seen as an invocation of the Nirvana fallacy.

The most dangerous part of the inversed stopped clock is that the crankery supported may be given much more weight than if it was supported by a known nutjob, due to the usually decent/sane reputation of the person promoting it. The opposite could also happen, and the person's one crank belief will become, again, mainly what they are known for, leaving the rest of their work marginalized by association. An example of the latter is Peter Duesberg and his HIV denialism.

A bit more cynical explanation is that everyone has a "need" to be powerful; this is the "pride" that is a "mortal sin". Ideally, this is done by actually having major accomplishments, such as establishing a business or completing a degree. In many cases, this desire can be filled by hyping up the importance of your hobbies or adopting "enlightened" worldviews. When fringe cases happen, this is done through buying into conspiracy theories or crank views, the "cheat codes" to knowledge. The person that "knows" that some dissolved silver cures AIDS is "smarter" than you or I. And while most of the people on this list do indeed have major accomplishments, they still feel the need to enhance their superiority, and it's often easier to adopt conspirational thinking or crank ideologies than it is to do something else important.