Hooray! @civodul merged my patches for Guile-ZLib (https://notabug.org/guile-zlib/guile-zlib/issues/4)
These patches fix decompression algorithm so it works properly on data with high compression ratio (e.g. when the input data is very uniform.)
I discovered the bug during my work on Guile-PNG (https://github.com/artyom-poptsov/guile-png): I was trying to decompress an image where most of the pixels were black, and Guile-ZLib kept telling me that there's an error in "uncompress" procedure.