Elusive String.blit exception

[Mike Hamburg <hamburg@fas.harvard.edu>]

I have a program which does not use the String module directly, which 
very occasionally fails on Invalid_argument(String.blit).  I have been 
unable to track down the cause of this bug, and really don't know what 
to make of it given that I'm not using the String module.  The only 
module that's used anywhere in that code which uses String (as far as I 
can tell) is Printf.  It seems very unlikely that Printf is failing, 
given that it's tested more often than just about any other module in 
the standard basis other than List.

Where could this exception be coming from?  Is there any way to 
automatically get a backtrace when my program dies on an exception?  
Are there any other functions which use String.blit?

Mike Hamburg