>>It is hard to reproduce on demand bug in SSMS. As a first step, try closing SSMS and re-start and re-try. If this will not fix the problem, then you can always attach DB using T-SQL.
>
>I closed and opened SSMS in the past few days probably 100 times. And I tried to attach using T-SQL and got a problem (I started a new thread on that issue).

I see. I read the thread William pointed to and it seems like the solution is to Run SSMS as Admin. Wondering why would this bug still not fixed - are you running RTM version and what is your SQL Server version?