Here's the general outline.
Vendor A writes a parser that is helpful and is liberal and infers missing quotes and stuff. Vendor B writes something that's mostly to spec, but accidentally doesn't properly quote things. It works fine, because A is liberal and infers these quotes.
Vendor C comes along and builds exactly to spec. But despite being perfectly to spec, it doesn't interop because B sends invalid data! But B is a big vendor, and their stuff works with A.
So now C must add a hack to their parser to deal with the fact that, because A was liberal, B got their implementation wrong.
One example is the loose routing parameter, "lr". It has no value, you just add the name of the parameter "uri;lr" in contrast to other parameters like "tag=bla". Some implementations send "lr=on", and that should be mostly harmless. Except other implementations take that to mean "lr" has a value, and no longer accept just "lr" as turning the feature on.
SIP is full of these things, many of them in the parsing layer alone, let alone actual semantics of what things mean. Browsers are another example: vendor A decides to allow closing tags out of order - how do you do handle such unspecified stuff cross browser?