Random database connection error

Question

Yesterday, a user of my qa site http://www.fotocomefare.com/domanderisposte/ reported the following error.

Could not establish database connection. Please check the username, password and hostname in the config file, and if necessary set up the appropriate MySQL user and privileges.

I saw it only once, before making it public. I and other users have already posted questions and answers with no problem.

What are the causes for this error and how can it be avoided?

Comments

I think this is not an q2a problem. May be there is something wrong with the server or hosting package. One reason to think that is, that the error does not show up always.

For me Your site works perfectly and it is as well very fast.