IE and FF show fav icon of one site on other site?

We are facing a weird issue and need some brain storming here. I will try to provide as much detail as possible.

One of our clients, let's say "Client A" has reported many a times now that the fav icon shown in their browsers (they tried with both IE & FF) is not correct. They say that the icon they see is of one of our other clients "Client B".

The problem is that it's an intermittent issue and we have never been able to replicate it at our end. We tried on multiple machines spread over different geographical locations India, US, UK, Germany, etc.

Our client "Client A" is from UK and the icon which they see is 开发者_运维问答of our client "Client B", which is from US & Germany. They have never seen any other client's icon. Its their own or Client B's.

We have multiple servers for load balancing and multiple Akamai servers for providing static content and all the servers are in US (although in the near future we will have a local server in UK & Germany).

We also checked the server logs to see if we could find anything weird, but nothing.

I guess it's an issue with caching, but how and where?

How are you differentiating with your web servers sending output to Client A versus sending to Client B? Presumably you have the same web server for both but you are not specifying any details. Can you be more specific about how the web servers are set up?

Or do you have the same web server, and are specifying favicon via HTML?