When an iPhone users sends a message to another iPhone user it is done via Apple's proprietary iMessage platform and users are identified by their telephone numbers. If you are a user that switches from an iPhone to Android without first disabling all iMessage logins on all your devices then you may end up in a situation where iPhone users can no longer message you. Anytime they try sending you a message, Apple will think you still have an iPhone and hold them until your iPhone comes back online (which may be never if you have migrated).

Apple seems to acknowledge the issue to Re/Code (link) with this statement

Users had complained that AppleCare agents were also unable to fix the issue but Apple says this limitation has since been resolved. As for what software changes could be implemented to fix this with a future update, we'll have to wait and see.