What happens:

If you use an empty bucket on a water source, the bucket will simply disappear if there is no room in the inventory to place the filled bucket.

What I expect to happen:

If you fill a bucket and it can not be placed in your inventory then I expected it to be dropped on the ground.

How to reproduce:
1. Obtain 2 empty buckets (stacked in v.7.2)
2. Fill the rest of your inventory with any items such there is no room new items
3. Load the bucket stack into your main inventory bar
4. Use one bucket to collect a source block
5. observe the loss of your bucket

I have been able to reproduce this on my android (4.1) phone as well.