Do you have ON ERROR routine which may be eating your error? Where is this code executed? BTW, you don't need SET EXCLUSIVE ON to use table exclusively. But it would give you an error if the table is used unless the error handler supressed this error.

>i have the following code which works sometimes and does not work at other times without giving any error.
>
>set excl on
>use tbsale2 in 0 excl
>sele tbsale2
>if type('tbsale2.ngroupname')='U'
> alter table tbsale2 add ngroupname N(6)
>endif
>
>since it works sometimes i assume the code is correct, a read only on the table should give me an error, however i get no error and the field does not get added, i am a bit lost here, any suggestions?