I recently ran into some issues with pretty paths and solr. Specifically I was getting a very ugly solr error when I tried to search by a string that didn't have a corresponding taxonomy pathauto alias. I traced it back to the taxonomy pathauto coder. When it decodes the path segment value, it just returns the value (string) if it doesn't find a taxonomy term for the path. This causes errors down the line because Solr was expecting an integer (term id).
My proposed solution is to return zero if the path lookup fails. Attached patch does just that.
Note: Currently I only know of this being an issue with Solr, however I feel that it could help avoid issues down the line with other search services.