But then, this is a well documented error and even we have attempted documenting it here - to save hours of debugging and, of course, to provide a good night's sleep!!
With the latest Atlassian Plugin SDK though, it seems the nightmares are back. And I have seen a rising number of questions about this error in Atlassian Answers and other forums. Are you one of those who ran into the same? If so, don't blame it on Atlassian or the plugin SDK. You are just ignorant, as I was for a good few hours, about the Atlassian Spring Scanner libraries.