I guess I can go first. I feel very disoriented. If I am focused enough, it can even be a physical sensation, like vertigo. Often, I will have to make the interrupter start over because I simply can not make a sentence out of the words until I willingly change focus. When this happens, it can become impossible for me to switch back afterward.
Oftentimes, I feel ADD-style symptoms afterward and become listless. If the interruption is breaking into something that was very important or something that has a very solid deadline, I can also have panic attack symptoms, such as feeling like my clothes are constricting, needing personal space, heart pounding, feeling like the room is "stuffy", etc. I think that I actually do panic a bit when I feel derailed, though, so I'm not sure if that counts.