Expenses usually decrease in retirement, and that is a good thing, because many retirees are getting by on half their pre-retirement income. But here is a disturbing fact: experts and retirees themselves readily identify six big expense categories that retirees should have set aside money to cover. But many did not.

And now they regret that big time.

Here are a half dozen big expenses that almost certainly will be part of most retirements.

Taxes. Retirement is not a free pass to stop paying taxes. Thirteen states tax at least some Social Security income: Colorado, Connecticut, Kansas, Minnesota, Missouri, Montana, Nebraska, New Mexico, North Dakota, Rhode Island, Utah, Vermont and West Virginia. Seven states do not tax pensions -- Florida, Nevada, South Dakota, Tennessee, Texas, Washington and Wyoming -- but that means 43 do. The IRS gets into this act, too, where Social Security income may be taxed if all income exceeds a specified base. The IRS also will seek to tax monies withdrawn from a tax advantaged savings plan. Many, many seniors assume they will stop paying taxes when they retire. It just is not so, and budgets have to be constructed with the tax man in mind.

Fun. Multiple experts pointed to this as a huge - and ironic - retirement planning oversight. The irony comes in, because many of us think of retirement as a blur of cruises, golf outings, multi-generational family vacations back to the ancestral stomping grounds in search of roots, and more. Just one problem: seniors probably have the time to travel but the other necessary ingredient is money. Of course, most have not set aside a budget to fund an annual vacation and other fun activities. What can be more boring than sitting around the TV watching reruns all day and much of the night because there is no cash for better activities? Plan in advance to avoid this tedious outcome.

Dental expenses. Medicare does not cover dental expenses and, sure, there are widely available dental insurance plans that are marketed to seniors. But check the coverage terms. A popular one caps coverage at $1,500 per calendar year and, of course, that's plenty for cleanings, maybe a filling, possibly an extraction. But talk about implants and dentures, and suddenly you are into five figures of work.

New car expenses. Average price of a new car now is over $30,000. That probably means $5,000 down, maybe $450 a month, maybe another $100 monthly for insurance, perhaps another $100 monthly for gasoline, car washes, oil changes. That can be a very big ding in a senior's budget when that senior has not prepared in advance to meet those expenses.

Hearing aids. One in three seniors has hearing loss at 65. Guesses are that one in four who need them buy them. That means three in four don't. Money is a big reason why many scorn them. (Vanity is the other reason.) Nope, Medicare usually offers not a dime of coverage for hearing aids. Prices have been falling due to technological innovations, but a pair still cost around $1,000 (down from $5,000 for the traditional technology). That can be big money for seniors on a tight budget and a fixed income. Word of advice: set aside money before retiring for hearing aids, also eye glasses if you wear them (prescription glasses often cost upwards of $500 and usually last just a year or three. Medicare rarely covers eyeglasses).

Health care. Most seniors applaud Medicare, it is a good program. But comprehensive it isn't. Fidelity Investments has estimated that a couple retiring at 65 will need $240,000 to cover future medical expenses. Even a skinflint will spend a lot out of pocket. Medicare Part A - hospital costs - has a deductible of $1,288. Part B - doctor expenses - has a $166 monthly deductible. That's $1,450 annually and that is just for starters. Stay in a hospital more than 60 days, and you may pay up to $322 co-insurance per day. The costs keep adding up, and that is why savvy seniors go into retirement knowing they will have to open their purse wide to pay for the health care they probably will need.