A fallibilist is always willing to consider that he could be wrong about anything. His attitude can be summed up by Karl Popper's credo (as quoted here):

‘I may be wrong and you may be right, and by an effort, we may get nearer to the truth.’

Note that Popper does not limit his credo. He doesn't say 'I may sometimes be wrong' or 'you may sometimes be right'. Again, we can always be wrong about anything. Knowledge is inherently uncertain and one should remain open to the possibility of being mistaken.

A crypto-fallibilist outwardly subscribes to Popper's credo but does not follow through on it. He lacks the requisite integrity. He may promote fallibilism explicitly, but he doesn't live it. At most, he is a fallibilist in some carefully constrained way. Crypto-fallibilism is a form of hypocrisy, a type of fraud: it's to pretend (to oneself or others) that one is a fallibilist.

The crypto-fallibilist eagerly reminds others of Popper's credo so that he can imply that 'you may be wrong and I may be right'. Worse, he has plausible deniability because he can always claim that the unmodified credo applies to him just the same as to others. He seeks to evangelize, even to spread fallibilism, ironically, but not to learn. He enjoys showing others when they're wrong but hates being proven wrong himself, at least in some areas. He doesn't discuss to find his own mistakes; he instead discusses to 'win', to show you that he is right and you wrong. He repeats fallibilist phrases – such as Popper's credo; 'don't immunize theories against criticism!' (a nod to Hans Albert); 'don't destroy the means of error correction!' (another to David Deutsch) – to associate with other cryptos.

While he explicitly agrees that he could be wrong about anything anytime, he refuses to consider that, say, he could be wrong about something he feels 'certain' about, that others might know some of his thoughts and motivations better than he does himself, or that he could be lying to himself. Just as an honest person will take seriously accusations that he has lied, even (or especially) to himself, a fallibilist will take just as seriously accusations that he isn't being fallibilist – but a crypto won't do that. His actions betray him.

Cryptos think that having read a few books by Popper or Deutsch is enough; that subscribing to fallibilism as a philosophy is enough. But it isn't. That alone doesn't make them actual fallibilists. Crypto-fallibilism is just an intellectual fashion, as Popper would call it.¹

In discussions, some crypto-fallibilists have derided my thoughts on the matter as 'purity testing'. But what is so wrong with taking Popper's credo literally and seriously, and judging accordingly? Some have also shrugged off these criticisms as them just 'fallibly being fallible'. That's a cop-out answer. While I agree that we can always only practice fallibilism and that we inevitably make mistakes, at times grave ones, even in our fallibilism itself, there's a difference between honest attempts on the one hand and reckless violations, lies, and evasions on the other.

There are some valid exceptions, such as protecting yourself from abuse or not talking to someone who you suspect wants to use what you say against you (legally or otherwise), eg not talking to the police without a lawyer. In such cases, you can reasonably claim that the people involved have already destroyed the means of error correction. Also, being a fallibilist doesn't mean others have a right to force you to discuss – you still have freedom of association. But compared to all the interactions one typically has, these situations are rare, and crypto-fallibilists will find reasons to continue being hypocrites more frequently than that.

Detecting crypto-fallibilism in someone isn't hard. I have found that it usually only takes maybe a dozen questions in a Socratic dialog to find something cryptos are not willing to challenge. That alone doesn't make them cryptos – most people are at least a little pigheaded – but if they nonetheless maintain that they're fallibilists, or if they say that fallibilism doesn't extend to that situation, that's an implicit confession that they're cryptos.

They may even say they admire Socrates, arguably the first fallibilist, but I suspect that if they ever met a real-life Socrates today, they'd hate him.

Because crypto-fallibilists are hypocritical justification-seekers, they are worse than regular justificationists. That's a shame since all it takes to avoid the hypocrisy is an admission that they're not true fallibilists. (Instead of putting 'fallibilist' in your Twitter bio, put 'aspiring fallibilist'!)

I'd like to be a fallibilist, but I don't yet know how. Slowly exposing myself to more criticism could be a way. I also want to practice asking Socratic questions and having more modest expectations in critical discussions. In some of my past discussions, I sought to evangelize; it's not surprising that I often failed at that, and I see now that the attempt was wrong. I was a crypto-fallibilist myself; that was a mistake.

Popper provides the following pointers:²

Serious critical discussions are always difficult. Non-rational human elements such as personal problems always enter. Many participants in a rational, that is, a critical, discussion find it particularly difficult that they have to unlearn what their instincts seem to teach them (and what they are taught, incidentally, by every debating society): that is, to win. For what they have to learn is that victory in a debate is nothing, while even the slightest clarification of one's problem – even the smallest contribution made towards a clearer understanding of one's own position or that of one's opponent – is a great success. A discussion which you win but which fails to help you to change or to clarify your mind at least a little should be regarded as a sheer loss. [...]
Rational discussion in this sense is a rare thing. But it is an important ideal, and we may learn to enjoy it. It does not aim at conversion, and it is modest in its expectations: it is enough, more than enough, if we feel that we can see things in a new light or that we have got even a little nearer to the truth.