The cap is not completely sealed which allows beer to get inside of it. It is sealed enough that makes it very hard to remove the trapped beer. I have tried many times and have been able to remove most of the trapped beer, but the beer that remains trapped creates a strong vinegar smell which affects the taste of the new beer. I can’t see where the beer is getting in and/or where it comes out when I shake it.
The issue appears to be caused because the cap screws into the inside of the growler compressing the liquid and pushing it into the space inside the cap. If the cap were made to screw on to the outside it would not create the compression that is causing the problem.
I like the unit overall, you just have to drink the new beer quickly in order to not have the taste change.
Brad 
