Wow! If you have money in a bank in Cyprus...

[cthulu2016]

23. Some rates changes can. New and/or unexpected taxes cannot.

re: And where does it say that in the Constitution?

In US law a rate change to an existing tax can be retroactive, but a wholly new tax cannot, particularly when, as is the case in Cyprus, it imposes a tax on an activity that was undertaken with the expectation of no tax at all (!)

If Cyprus were the 51st state, under the 5th Amendment the Cyprus solution would be struck down.

Blodgett v. Holden and Untermyer v. Anderson struck down the retroactive application of the federal gift tax, establishing the "wholly new" standard.

The most recent judicial description of the standard is that a “wholly new tax” is a tax that “could not reasonably have been anticipated by the taxpayer at the time of the transaction.”