I am always amazed when people fail to distinguish between constants and variables. I am all the more amazed when the victims of such confusion are the otherwise brilliant implementers of programming languages. You'd think that if anyone knows the difference between a variable and a constant, it would be a programming language implementer.
For instance, a CLISP maintainer explicitly based his argument for making some backdoor compulsory on the belief that the behavior of his hypothetical source-closing adversary will remain the very same after the backdoor is created. But what is constant here is the hypothetical adversarial will of said antagonist, not his behavior; the known backdoor will be trivially circumvented by this adversary, and will only remains as a constant hassle and security hazard to all the friends.
In another instance, the respected creator of Python argued against proper tail calls because they allegedly lose debugging information as compared to recursion without tail call elimination. But as said hacker implicitly acknowledges without making the explicit mental connection, in programs for which proper tail calls matters, the choice is conspicuously not between proper tail calls and improper tail calls, it is a choice between proper tail calls and explicit central stateful loops. And the absence of debugging information is constant when you transform tail calls into stateful loops. Stateful loops precisely make it harder to get debugging information, whereas proper tail calls are trivially disabled, wrapped or traced (and trivially so if you have macros). In addition, state introduces a lot of problems because of the exponential explosion of potential interactions to take into account. But more importantly, proper tail calls allow for dynamic decentralized specification of a program in loosely coupled separate modules by independent people, whereas loops force the static centralized specification of the same program by a one team of programmers in one huge conceptual gob requiring tight coupling. Finally, loops are trivially macro-expressible in terms of tail-calls (i.e. through local transformations), whereas it requires a global transformation to transform arbitrary programs requiring tail-calls into programs using loops - and if we allow for such transformations, then who needs Python, INTERCAL is the greatestest language ever designed.
Brilliant operating system designers have argued that microkernels can simplify software development because factoring an operating system into chunks that are isolated at runtime allows to make each component simpler. But the interesting constant when you choose between ways to factor your system and compare the resulting complexity is not the number of components, but the overall functionality that the system does or doesn't provide. Given the desired functionality, run-time isolation vastly increases the programmer-time and run-time complexity of the overall system by introducing context switches and marshalling between chunks of equivalent functionality across the two factorings. Compile-time modularity solves the problem better; given an expressive enough static type system, it can provide much finer-grained robustness than run-time isolation, without any of the run-time or programmer-time cost. And even without such a type system, the simplicity of the design allows for much fewer bugs, whereas the absence of communication barriers allows for higher-performance strategies. Hence HURD being an undebuggable joke whereas Linux is a fast, robust system.
In all these cases, the software designer enforces some kind of hurdle that doesn't help honest people; the only beneficiaries are the specialists who get job security at handling the vast increase in gratuitous complexity.
These people, though very intelligent, fall for an accounting fallacy. They take a myopic look at the local effect of one alternative on some small detached parts of the system where they can indulge in some one-sided accounting whereby the alternative they like has benefits at no cost, whereas the other one has costs at no benefit. And they neglect to consider the costs and benefits in other parts of the system outside of their direct focus though they are necessarily being changed by the switch between alternatives to preserve the actual overall constants that make the choice meaningful.
It is possibly the individual interest of these experts to promote labor-intensive solutions where their expertise is the development bottleneck. Conscious dishonesty isn't even necessary when the rational incentive is for the experts to ignore the real costs of their choices, because they don't bear these costs. And so ultimately, the laugh is on the users who follow the advice of these experts.
In a choice between proposed alternatives, what needs be evaluated is the economic cost of each alternative, i.e. its relative cost to other alternatives with respect to the overall system. And before you may even evaluate this cost, you must determine what is constant and what varies when you make the choice.
Woe be on software designers who confuse constants and variables!