for RSS feeds that link to a proxy (for authentication, or logging, ...), e.g.:
which the proxy will redirect to the article:
The article URL is a query param, so it's correctly percent-encoded. However, in /lib/simplepie/simplepie.class.php, line 11402 has a section that decode the percent-encoded characters blindly, causing the proxy not being able to parse the URL.
This bug is not in simplepie 1.2.1, only in 1.3-dev. It's ironic that the function named replace_invalid_with_pct_encoding(), yet it also did the reverse.