Recently, an issue was identified with joist families in Revit Structure 2009 & 2010. Users find that an error comes up if a joist is drawn shorter than the minimum length for a joist family. For example, say your drawing a joist using the 24LH05 joist family. The shortest valid length is 20'-0". This is because in order for the family to be valid, it requires a set of internal panels with a variable and end panel on each end, see the image below for clarification.
Though the types of panels may change from one family to another, the concept applies to ALL joist families built with panel arrays or groups, including: K-Series, LH-Series, SLH-Series, DLH-Series Bar Joists and Joist Girders.