Have a glass bottle in Creative and fill with a water source block/cauldron, it gives you a water bottle, but also removes the empty glass bottle, which is inconsistant with Creative mode behaviour of keeping the item in hand.
Note: Right-clicking with more than one glass bottle in hand does not consume the bottles when filling from a water source only.
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To be consistent with other Creative mode behavior, the item in your hand should be unaffected. In this case, the Glass Bottle should remain in the same slot on your hotbar, and a Water Bottle should be created elsewhere in your inventory.
I think that's probably what Tails was saying, but I didn't find it particularly clear.
Exactly 🙂

Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

Is this still a concern in the latest Minecraft version 14w08a? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

In 14w34c, this issue is reversed, likely works as intended. No water bottle is given, the glass bottle remains, but the water in the cauldron gets consumed.

Relates to MC-124280
The "duplication" of water bottles matches the creative mode behavior of not using the item you hold, so I would say the glass bottle beeing replaced by the water bottle is the main bug here. It would also be more convenient if the water bottle would stay, that way you could fill up your inventory with water bottles really fast.