Can reproduce this both in Schema Browser and in the Describe window. Sometimes I get an access violation, sometimes TOAD just disappears.
CREATE TABLE toad_test AS ( SELECT ROWNUM AS rn FROM dual CONNECT BY ROWNUM <= 1000 );
ALTER TABLE toad_test ADD CONSTRAINT pk_toad_test PRIMARY KEY ( rn );
Open the table in Schema Browser or describe it (F4).
Duplicate a row (will violate primary key once trying to insert).
Use the mouse to move the vertical scrollbar.