I recently uncovered an overflow problem in BDC columns. When returning data from a BDC data source, it is possible to return too much data if the BDC is being used as a column in a custom list. The maximum length of a BDC column when used in a list is 255 characters, which is the default maximum length of a text type column in MOSS. When using the BDC web part, data that is returned is not subjected to the maximum length of 255 characters. When more data is returned than 255 for a particular column, I received the error: "Invalid text value". Within the BDC application definition, the TypeDescriptor data type for these columns was System.String. I could not find any other errors in the event logs or SharePoint logs for this problem.
I wanted to post this maximum length because the MSDN documentation does not mention this.
Since I was working with a SQL Server BDC connection, I was able to wrap fields that could overflow the 255 character maximum using the substring function as in substring(<fieldname>,1,255), but this will not be possible for all BDC connections.