You can't define syntax extension that starts with #f and is followed by symbol

[jcubic]

So the problem is this:

(set-special! "#foo" 'frob lips.specials.SYMBOL)

#foo"xxx"

#foo"xxx" is parsed as a single token, and lexer thinks it's a symbol.

The problem is these rules:

    // hash special symbols, lexer don't need to distinguish those
    // we only care if it's not pick up by vectors literals
    [/#/, null, /[bdxoeitf]/i, null, Lexer.symbol],

Those are exceptions to hash (vector literal) to handle special tokens. They need to added and sorted together with specials.