Personally I blame Gandalf, there is a reason that no-one dares utter the black speech in Imladris. I am convinced that was what caused the trans dimensional anomaly that led to all the chaos here in Middle Earth.
I think we can leave aside the earlier incident in which Thorin and his company met seven other dwarves. (Note we must find a collective noun for dwarves I can't keep referring to them as a bunch or a shower). Anyway the larger bunch, with accompanying hobbit, were on their way to the Misty Mountains when they encountered the smaller shower going in the opposite direction singing Hi Ho! I understand Kili had to be physically restrained from going with them when he found out there was a rather attractive dark haired princess living with them. This incident is just one of these odd malfunctions that turns up from time to time and our technical people tell me it's very difficult to diagnose an intermittent fault.
So onto the main problem (sorry are we calling these issues these days?) people and objects from other dimensions turning up in the middle earth universe and in some cases objects and people going the other way but more of that later.
I will be using my new I-Palantir with the record and playback facility to demonstrate some of the 'situations'
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The Council of Elrond
I said it was probably Gandalf's fault but we might have got away with it if it weren't for Boromir and his [expletive deleted] horn. As you know horns of any kind can act as summoning devices and the horn of Gondor appears particularly potent. He no sooner blew the damn thing than Frodo who was trying to find out which way he should go was surrounded by a host of munchkins telling him to 'follow the yellow brick road.' And that as they say was only the start.