I have been using the Singleton for awhile now and ran into unexpected results with an app recently.

I most often used the singleton to get an instance of a class throughout several parts of a framework I wrote. No other method other than the singleton instance was static.

When I used it, it would return the wrong results. For example, an authorization controller I wrote, upon checking if a used was logged in ( a simple bool ), would always return false. Then I tried this...

which fixed the problem. If it helps, I also had a factory method for chaining and what not. I know the singleton was setup right to return the instance, I just don't know what was causing the issue and still haven't figured it out. Maybe authorization was just a bad idea for a singleton, but either way its working quite well now.