I know it is a single example and we should extrapolate much out of it, but in the case of html those who accepted more liberal input (html4/5) won over over those that were more conservative (xhtml).
But that doesn't really apply to protocols like TCP. Postel's "law" is best understood in the context of 1980, when TCP had been around for a while but without a real standard, everyone was kind of experimenting, and there were tons of little incompatibilities. In this context, it was reasonable and practical advice.
For a lot of other things though: not so much. "Fail fast" is typically the better approach, which will benefit everyone, especially the people implementing the protocols.
This is also why Sendmail became the de-facto standard around the same time by the way: it was bug-compatible with everything else. Later this become a liability (sendmail.cf!), but originally it was a great feature.
I also read that XHTML made template authoring hard, as the template itself might not be valid XHTML and/or different template inputs might make output invalid. (I sadly can't find the source of this point right now, but I can't claim credit for it).
With PHP specifically there was an issue where the use of shorthand <? syntax for code snippets would conflict with <?xml declaration that would normally be placed at the beginning of the XHTML document - it would see the <? and try to interpret the rest of it as PHP code, which obviously didn't work. The workaround was to disable short tags and always use <?php explicitly
Also note that HTML5 codified into liberal acceptance some of the "lazy" manual errors that people made in the early days (many of which were strictly and noisily rejected in XHTML, for example).