It seems we depend each other more that we think.
In our test environment we are using flashback technology. This technology afford to us to keep our test servers in the actual state regarding to production server, and the same time give the opportunity for testers make their tests for the new features for our system. We can roll back all changes which has made our testers, and roll forward all new changes which has made on primary server without restore whole database. This is very convenient. Couple days ago my colleague asked me if I can convert snapshot standby to primary without roll back changes which has made on this standby. This is important point, because in a normal way if you want to convert snapshot standby you have to flashback your database, and roll back all changes. But we can escape this.
Some times you need to perform restart standby database to apply some system changes. And here you can catch some pitfalls.
When you try to perform start up standby database in open for readonly mode, you can see this error in your alert log.