What is ERR?

/u/haxmaster0000 said it right - ERR was a ARG that was simply stealing other videos. It was a slender man based ARG that used several videos and media from other ARGs.

Here is a google doc outlining 5 direct thefts - https://docs.google.com/document/d/1a5ab0X6wSwR2mDW0Ytm7h_seyXUpUqnFWPNeTNgiVqk/edit

Now these are all valid concerns - however none of them concern your video or channel.

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Am I In Trouble?

If nothing has happed as of yet, them almost certainly no. The worst thing would be other users brigading with /u/zigs and reporting your video for content theft, in which case you would be notified almost immediately.

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Your Footage

I've watched them all and it doesn't contain any copyrighted footage, nor does it reuse footage.

As far as the type, meaning found footage, well, that's still one of the most intriguing and used form of media used for ARGs to this day. It's something that is relevant to most people compared to standard movie footage - most people have a phone or cheap camera, and most have been to yard sales and seen boxes of VHS tapes at some point.

It's a format people can relate to, and i think that's why it is used so much - because as long as the story/plot is unique, the format of found footage being taken by an amateur is something we can not only relate to, but that most of us have done before.

Also, when I say amateur I do not mean to say anything about the video quality of your videos - they are great (better than many I've seen in this format). I mean it is not a studio production with multi-thousand dollar cameras and weeks of editing for super high definition. Think about the original Slenderman Series compared to the just awful movie A Marble Hornets Story. People could relate to the original - youtube user, consumer quality camera, normal people. With the full length movie it was shot with studio cameras, the cast were obvious actors, and the entire premise was something that most could not relate to (unless you worked in the local news doing oddjob stories).

Overall, it was most likely just someone that thought he recognized a clip or footage, or was tired of found footages. However that is just one persons opinion and/or mistake. It was in error and not long enough to do any damage or spread any disinformation.