Noticed during various CMO rounds that using the hypo with a bottle bag, you cannot inject back into a bottle or a vial. I believe drink containers like cans and water bottles are also unable to be injected into with the hypopen.
You can carry a beaker to inject back into and pour it back into the bottle/vial in your inventory.
But being unable to inject into these types of containers feels like an oversight and might be a good QoL for Hypo users.