In short, you should expect the unexpected, but we'll try to minimize that here. Retiring in Europe as a U.S. citizen can involve navigating a variety of unusual rules and regulations, which vary by country. Here are some of the less common or unexpected rules you might encounter:

Minimum Income Requirements: European countries require proof of sufficient income to support yourself during retirement before granting you a long-stay visa. This can range from a modest amount to a significant sum, depending on the country. For instance, Portugal has relatively low income thresholds (currently around $850/month) you must meet to obtain a retirement visa. While Switzerland is on the high end (currently around $9,500/month). Other countries with modest income requirements include Spain, Italy, Slovenia, Greece, and Malta.

Health Insurance: Most European countries require proof of comprehensive health insurance that covers you in their country. This can be a private insurance plan or participation in the national healthcare system if you qualify. If you shop around, you will be amazed at how much cheaper this is than insurance in the states.

Taxes: Understanding the tax implications of retiring abroad is crucial. Some countries have double taxation treaties with the U.S., which can affect how your income is taxed. Portugal, for instance, offers a Non-Habitual Resident (NHR) tax regime that can provide significant tax benefits for foreign retirees. Unfortunately, each country has their own tax rules irrespective of their membership in the E.U. Whole books are written on this subject.

Property Purchase Restrictions: Certain countries have restrictions on property purchases by non-EU citizens. For example, in Denmark, Switzerland, Austria, Iceland, and Hungary non-EU citizens must obtain permission from the appropriate jurisdiction to buy property. In other countries, like Bulgaria, you can buy apartments, but not houses with land.

Pension Transfer: Transferring U.S. pensions to Europe can be complex. Some countries have agreements with the U.S. that facilitate the process, but understanding how your Social Security and other retirement benefits will be treated is essential. It is often easier to maintain your U.S. accounts than deal with European bureaucracy.

Integration Requirements: Some countries, like Switzerland, require proof of integration into the local community, which might include participation in local events or organizations. Language courses may also be required. German speakers are particularly fond of their language.

Age Requirements: Some countries have minimum age requirements for retirement visas. For example, Malta has a retirement scheme specifically for individuals over the age of 55. However, most countries have an open interpretation of "retirement age" as long as you have a stable or passive income source.

These rules can change, so it's essential to consult with an immigration attorney or a relocation specialist familiar with the specific requirements of the country you're interested in.