>My boss has determined that the problem is somehow connected to a corrupt ESL.
>He copied a "good" ESL over the one he questioned, and the problem went away.
>The question remains, what happened to the original ESL to cause this?
>

A cow flew by and left a small dropping on the original file. You might consider taking the step of flagging the file as read-only in the OS (it can be done programmatically through calls to the Win32API, as shown in the Files Section, or through calls to the WSH's Scripting.FileSystemObject (see my sig block for references.) Even with these precautions, it'sossible for a file's content to get corrupted in an undetectable way through soft disk errors (the chances are vanishingly small that undetectable corruption will occur) but it can happen.