What are the hazards of upgrading a SQL Server service pack?

Question

Are there any specific pitfalls/standard considerations when upgrading an instance of MS SQL Server to a higher service pack? Is there anything special that is generally done besides making sure there are current backups of system and user databases?

I'm asking the question in general since I presume the answer is similar regardless of version or situation. However, in the case that I'm mistaken and the specifics do matter, the situation that prompts my question is an upgrade of SQL Server 2008 from SP2 to SP3, running on Windows Server 2008 with failover clustering.