for example, the following produces the indicated error:

$ echo '```raw
hi
```' | pygmentize -l md

*** Error while highlighting:
TypeError: cannot use a bytes pattern on a string-like object
   (file "/home/nick/src/pygments/pygments/lexers/special.py", line 87, in get_tokens_unprocessed)
*** If this is a bug you want to report, please rerun with -v.
```raw

I believe this happens because the markdown lexer will parse the fenced code and see "raw" which invokes the RawTokenLexer which I don't think is meant to be used like a normal lexer (see https://github.com/pygments/pygments/blob/master/pygments/lexers/special.py#L45-L49)

You can see this in the wild using this README.md:

https://github.com/netdata/netdata/blob/master/collectors/perf.plugin/README.md

This repo has nearly 50k stars and over 4.5k forks so it seems pretty reasonable to handle this kind of issue.