The buckets of water problem
The problem is NP-hard, by reduction from SUBSET-SUM. Given a multiset of numbers $x_1,\ldots,x_n$ and a target $T$, consider $n$ buckets with capacity $C=x_1+\cdots+x_n$, initially filled with $x_1,\ldots,x_n$, and ask whether you can obtain a bucket filled with exactly $T$. You can prove by induction that all buckets are always with filled either with $C$ or with the sum of some (possibly empty) subset of the $x_i$s.