I recently made a mixed berry cider using 5.25 gal of store bought apple juice, 1 gal of Old Orchard Mixed Berry Juice and 3 lbs of Cherry Berry Blend frozen fruit. In the past, I've added the fruit after a week in Primary. This time I added it when I pitched. When I checked on it as I was heading out the door to work the next day, I found that the stopper and airlock had been blasted out along with some berries. I wiped off the lid, sanitized the stopper and replaced the broken airlock. Work got so hectic that I didn't do anything with it for 5 weeks.
When I finally transferred it to Secondary, it was very cloudy, very fruity, and very sour. SG was 1.000. After two weeks in Secondary, SG was still 1.000. It had cleared some, but was still hazy. I stabilized and racked into a keg, onto 4 cans of frozen raspberry apple juice concentrate. After cooling and carbing, it was still really sour, so I made a simple syrup from one cup of the OO Mixed Berry Juice and two cups of morena sugar and added it to the keg. It's very tasty, now.