They are warning that you should build at your own risk . If you keep additional Nursery Barns (over 2) on your farm, it is possible that you may lose any animals stored in additional Nursery Barns whenever the glitch is patched in the future.
Update: The June 14th FarmVille Official Podcast announced that they will allow farmers to keep up to two Nursery Barns, but any additional Nursery Barns over two may be lost when the glitch is patched.
FarmVille Community Manager, Lexilicious announced via Official FarmVille Forum the following statement:
This article originally appeared on FarmVille Freak."Hi Farmers,
Since the release of Nursery Barns, a glitch has come to our attention that allows users to have more than one Nursery Barn.
This is not the intended functionality of this feature and the ability to have more than one Nursery is a glitch.
We are intending on fixing this issue in the future, however we have decided to allow users to have the ability to possess two Nursery Barns on their farm.
Please know that if you continue to take advantage of this glitch and choose to have more than two Nurseries on your farm, you are building at your own risk, as this issue will be addressed in the future.
This fix may result in the loss of any Nursery Barns in excess of two.
We appreciate your patience and understanding with us during this time.
Happy Farming!" (Source: FarmVille Forum)