During this question about "China Gates" by John Adams another question came up:
What is the meaning of the little vertical line in bass clef bar 4 in front of the C?
The same mark re-appears several times in the piece, for example in bar 19 in front of an E. Thus, I assume it is not an error.
My first guess was, it was some uncommon accidental, but I could not find it in the related Wikipedia article.
The piece alternates between different modes: A-flat mixolydian, G-sharp aeolian, F lydian and F locrian (source). Could this play a role here somehow?