Some characters that are valid in filenames but invalid in URLs aren't handled properly by the url_for method. For example,
a call

AWS::S3::S3Object.url_for('/path/to/a&b.pdf', 'my_bucket') 

produces something like 

http://s3.amazonaws.com/my_bucket/path/to/a&b.pdf?AWSAccessKeyId=...

The "&" character is invalid in the path component of the URL and it should be escaped:

http://s3.amazonaws.com/my_bucket/path/to/a%26b.pdf?AWSAccessKeyId=...