Consider the following file snippet:
<table:table-cell table:style-name="Gnumeric-default" table:formula="of:=IF(RAND()>0.5;"#DIV0!";1/0)" office:value-type="string" office:string-value="#DIV/0!">
There appears to be no way to determine whether the cell currently contained an error or a string. (Note that for the given expression both is possible.)
If the cell format yields different results for errors than strings (for example in Gnumeric the default alignment differs), then this file can not be properly displayed.