Once I did a request with a valid API key

http://api.stackoverflow.com/0.8/stats?key=MYKEY

Every requests (from that IP) later on without key will use existing session with API key, limit counter keep decreasing.

http://api.stackoverflow.com/0.8/stats

For example, If the user use one of the proxy with a key and any other people who use later on that proxy with no key, they will get with remaining 10k limit just like with API Key access. Then it will be serious case right? We cannot limit users not to use proxy also.

So, Please don't allow requests with No Key to use existing session with API Key. Just let them use per IP based sessions.