We ran some front-end tests and saw the error:

Syntax error, unrecognized expression: #

We dug for a while looking for a stray # in our code before searching the internet deep enough to realize this was an invalid jQuery selector.

This error message would be more clear if it mentioned "sizzle", "jQuery", "selector", or another string that would make it clear to the user that this is not a JavaScript syntax error, but a sizzle selector expression syntax error.