What is the "misbehaving" log message? I was looking my bitcoind log and I saw this message:

Misbehaving: 91.230.200.131:8333 (90 -&gt; 100) BAN THRESHOLD EXCEEDED


In which cases this message is shown and why?

It just a curiosity.
If some client's actions do not correspond with the Bitcoin protocol in a disruptive manner (e.g. flooding with invalid messages) a ban counter is increased depending on the seriousness of the violation of the protocol. When the ban meter reaches some predefined value (can be set with the -banscore option in bitcoind, default is 100) connection to that client is interrupted for a day (that's the default at least, but again, this can be changed with the -bantime option).

The values used are arbitrary (e.g. 100). It all about relativity. I don't know exactly how much penalty you get for which violation, but this can be found in the source code. If someone feels the need to compile a list of the amount of penalty per violation, please edit this answer.