On Sat, Sep 11, 2021 at 4:56 PM Eric Timmons <etimmons@mit.edu> wrote:

    colonsym     = ':' | signsym

Definitely a thinko on my part: fixed.

The implementations
all seem to treat the character sequences #\\ #\n and #\\ #\t as
#\Newline and #\Tab, respectively.

Other people's thinkos: the spec prevails.  If you could make a PR for the CL version to eliminate them, that would be handy.

Additionally a #\\ not followed by an
#\n #\t #\" or #\\ will signal an error.

I think it's appropriate to signal an error on every backslash not followed by a quote or another backslash, as they are POSE syntax violations, just as a stray ) or a # outside a string would be.
 
Third, I'm not sure how to deal with the case sensitivity of symbols. Is
there any plan to add a :case argument or similar to POSE:READ? 

The problem is deeper than that: if CL's POSE parser receives (Foo foo), it will lose the distinction.  I am arguing (in #s-expressions in the libera.chat IRC network, if you want to join in) that since symbols are basically labels for things, we don't really lose anything by making them case insensitive.