Yes a common pitfall that people fall into is not allowing enough space on their SharePoint servers. Say you estimate your content database are going to get to 100Gb then you actually probably need 420Gb of space - how so...

 Content databases: 100Gb

 Search Index database: 40Gb (This is usually 20-40% of the size of the content databases)

 You need only a 50% fill factor on SQL drives to allow for SQL maintenance operations to run, so therefore 140Gb becomes 280Gb. And presumably you are going to back all of this up, so that is another 140Gb! Tada we have 420Gb!