Voltage between two terminals of an open circuit which's connected in series with a current source
For an ideal theoretical current source with open terminals, you could consider the potential between to be infinite*.
*(though as Phil notes, it is an "impossible" circuit so the answer is meaningless really - an ideal current source has a constant current, so you are asking a current to flow through an infinite resistance which makes no sense)

Since \$ V = I * R \$ it follows that \$ I \cdot \infty R = \infty V \$

Example in SPICE with a 1A ideal source and the resistance increased from \$ 0 \$ to \$ 1G \Omega \$: