Sporadically, hovering on the info icon of a Reference field in IE8 is generating a "'null' is null or not an object" error when a client script is in place that runs an AJAXEvaluateSynchronously evaluation that returns a True value. This appears to be most easily reproducible when mousing over a number of reference fields. The issue can be reproduced in IE8.
Rework your script to use getXML() rather than AJAXEvaluateSynchronously.
Related Problem: PRB599987