I've found two strange behaviours when using the clay buckets with placed fluids from other popular mods.
#1

Picking up either Build-Craft fuel or oil with a hardened clay bucket will cause it to transform into an iron filled bucket.
#2

Picking up any liquid placed by Mine-factory Reloaded will cause it to transform into a hardened clay water bucket.

Picking up sewage resulted in a vanilla bucket of water