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If you place a portal in the stairway to the arena of a Cold River map (in the small region that becomes inaccessible during the boss fight) and then die mid-boss fight, future attempts to enter the map will give you "An unexpected disconnection occurred" after a couple seconds of loading. This happens on every attempt. Given that this is a fairly convenient-looking place to drop a portal, it should really move your portal outside when starting the boss fight or at least state that you can't use portals there.
Ref #: 3,411,179,033
Thanks for your report, we'll look into this.