Guestbooks are a part of your Habbo User Page. People can leave messages on it.
As a result of a technical response to an issue in which guestbooks with over 1000 posts would cause the site to crash,  the guestbook post counter now shows ">1000" if a guestbook exceeds 1000 posts. This was implemented on November 2009.
Leaving three messages within a minute (60 seconds) will lead to a ten-minute ban from signing anybody's guestbook. This system is put in place to prevent spam, however the 3 messages left within a minute may not be spam, leading to much confusion.