I was under the impression that by now everyone agrees this maxim was a mistake. But I recently saw this answer which has a "be lenient" comment upvoted 137 times (as of today).
In my opinion, the leniency in what browsers accept was the direct cause of the utter mess that HTML and some other web standards were a few years ago, and have only recently begun to properly crystallize out of that mess. The way I see it, being lenient in what you accept will lead to this.
The second part of the maxim is "discard faulty input silently, without returning an error message unless this is required by the specification", and this feels borderline offensive. Any programmer who has banged their head on the wall when something fails silently will know what I mean.
So, am I completely wrong about this? Should my program be lenient in what it accepts and swallow errors silently? Or am I mis-interpreting what this is supposed to mean?
The original question said "program", and I take everyone's point about that. It can make sense for programs to be lenient. What I really meant, however, is APIs: interfaces exposed to other programs, rather than people. HTTP is an example. The protocol is an interface that only other programs use. People never directly provide the dates that go into headers like "If-Modified-Since".
So, the question is: should the server implementing a standard be lenient and allow dates in several other formats, in addition to the one that's actually required by the standard? I believe the "be lenient" is supposed to apply to this situation, rather than human interfaces.
If the server is lenient, it might seem like an overall improvement, but I think in practice it only leads to client implementations that end up relying on the leniency and thus failing to work with another server that's lenient in slightly different ways.
So, should a server exposing some API be lenient or is that a very bad idea?
Now onto lenient handling of user input. Consider YouTrack (a bug tracking software). It uses a language for text entry that is reminiscent of Markdown. Except that it's "lenient". For example, writing
- foo - bar - baz
is not a documented way of creating a bulleted list, and yet it worked. Consequently, it ended up being used a lot throughout our internal bugtracker. Next version comes out, and this lenient feature starts working slightly differently, breaking a bunch of lists that (mis)used this (non)feature. The documented way to create bulleted lists still works, of course.
So, should my software be lenient in what user inputs it accepts?