...Thanks! But, why did it work?

When OPTIMIZE is on VFP tries to be as fast as it can. As cursors normally are readonly, it can use a trick if the data is requestet in the same format as the source (i.e. no calculated fields, no grouping etc).

so if VFP finds a query like this:

select *;
  from SOURCE;
    where CUSTNO > "1234";
     into cursor DEST_1

it actually does this:

  select 0
  use SOURCE again alias DEST_1
  set filter to CUSTNO > "1234"
  

if it now would allow another ... INTO CURSOR DEST_1, You might ruin Your original data.

You can check by issuing

? dbf( "DEST_1" )

after the query.

So if You really need a cursor to further work with use the NOFILTER clause as the others said, to tell VFP "I do need this for furter work".