When trying to fill an empty bucket with either water or lava from a source block, the bucket remains empty. If only one source block is present, it disappears as if it filled the bucket, but the bucket still remains empty. This also affects catching fish in buckets as well.
Edit: this seems to only be a problem in creative mode, they're working just fine in survival
This is how it's supposed to work in Creative mode, see MC-9856.