Here's the Reproduce steps,
Change DataType of Column from varchar(xx) to nvarchar(xx)
click ok or goto script tab
50% of the time clicking okay will work
50% of the time, it mentions, it cannot delete tmp.* table due to FKs (I assume toad rebuilds the table as a tmp table then drops the old and renames the tmp.* table or something similar). If this one happens the tmp.* will remain and be visible. at this point manual clean up of this table is required.
See 2 screen shots where 1 shows the column dataType change.
and 2nd shows the error message. Looks like a weird Korean collation? not sure I'd have to look that one up