AOL’s chief executive, Tim Armstrong, drew plenty of attention earlier this month when he seemed to attribute a change in the company’s 401(k) plan in part to a couple of employees whose infants required expensive care. But what was mostly lost in the discussion was just how much it would cost employees if every employer tried to do what AOL did.

The answer? Close to $50,000 in today’s dollars by the time they retired, according to calculations that the 401(k) and mutual fund giant Vanguard made this week. That buys a lot of trips to see the grandchildren — or scores of nights in a nursing home.

Mr. Armstrong ultimately reversed the change after the uproar over the singling out of particular employees. Still, everyone who saves in a 401(k) or similar plan needs to take a close look at what AOL was trying to do, so they can recognize it and protest if their employer tries to do something like it. While there are plenty of federal regulations governing the basic administration and safeguarding of employer-provided retirement accounts, companies have a lot of leeway to alter their own plans in ways that can cost employees plenty. AOL’s attempt is an unpleasant reminder that employers can and will make changes to employee benefits programs for any reason at all.

Most companies that match your contributions to a workplace retirement account deposit that match each time you get a paycheck. AOL wanted to wait until the end of each year and deposit any match all at once. People who left the company during the year, by choice or by layoff, would have gotten no match at all, not even a prorated one.

Not many companies have adopted these so-called last-day rules so far. As of the end of 2011, just 7 percent of clients at the benefits consulting firm Mercer deposited their 401(k) matches annually. Aon Hewitt’s 2013 study put the number at 8 percent. The latest Plan Sponsor Council of America survey puts the figure at 17 percent.

Last-day rules tend to be a bit more popular among banks. There, however, it may not hurt employees quite as much, since employees tend not to walk out under their own power until after they’ve gotten their year-end bonus. If they hang around long enough to cash that check, their match will have already cleared. (Midyear retirees usually get their matches as well.)

The Disadvantages of a Lump Sum 401(k)

A small percentage of companies distribute 401(k) matches to their employees in one lump sum each year, rather than contributing to each paycheck throughout the year. This system can be detrimental to employees’ 401(k) balances if they miss some years’ matches when they change or lose jobs.