Base64 encoding Allowed Characters
If you have a proper encoder for Base64, you will not see special characters except:
[A-Z][a-z][0-9][+/] and the padding char '=' at the end to indicate the number of zero fill bytes
There is another Base64 character set available which replaces [+/] by [_-] making the encoding URL-safe.
Nevertheless the specification allows to include any other character. Often the Base64 encoded data contains a line feed '\n' every 76 characters. Any character except the ones mentioned above has to be removed during decoding. The padding characters indicate the number of zero bytes appended to apply to n*4 output characters.
transforming "weird" and non printable characters is kind of the whole point of base64, so no, you wont see those. more info here http://email.about.com/cs/standards/a/base64_encoding.htm