MUTUAL Intelligibility 

I've seen many people say that they can understand a different language on the discussion page and then list the languages as mutually intelligible. That is just one-sided intelligibility. Intelligibility differs from person to person and also depends on what is being said. For instance, if you're Russian you may understand a Polish guy completely, but your Russian friend may have no idea what's going on. And if the Polish guy says something else, you might not understand him at all. It is very hard to study intelligibility because there is so much variation. So just because you understand (or don't understand) something in a related language it does not mean that is the case for all speakers (or even most speakers) and it definitely does not guarantee it goes the other way around.
Mutual intelligibility is not one guy being able to understand something in another language. This is an example of mutual intelligibility:

A Polish guy is in the Czech Republic and asks a Czech guy for directions (in Polish). The Czech understands what he is saying and tells him where to go (in Czech). The Polish guy understands and finds where he needs to go.

If the Polish guy didn't understand then it is not mutual intelligibility. And this has to happen for a lot of people, not just those two guys. Alex