I have been using 3D touch to switch apps since I got my new iPhone. (View this gif to see app-switching in action.)
A few days ago I learned that I missed a couple of important bills. I use Fantastical to save and process most of my reminders, and it turns out that all the checkboxes are at the very left of the screen, which is about where I'd have to press to switch apps.
Just now I was in the process of switching apps and I accidentally tapped a checkbox for one of my reminders. Eureka! Perhaps I checked off some of my bill reminders while I was switching apps, so they disappeared from my list.
Lesson learned. From now on I will have to double-tap on the home button while I am using Fantastical to make sure this doesn't happen again.