The company said that the S&P 500 index gained 0.1 percent and the MSCI EAFE index lost 3.6 percent in June. Typical discount rates for pension plans as measured by the Mercer Yield Curve decreased by 28 basis points to 3.47 percent.

"The events in June provide a reminder of how quickly pension funded status can move when there are shocks in the market," Matt McDaniel, a partner in Mercer's retirement practice, said in a statement. McDaniel added, "In the two trading days following the Brexit vote, we saw funded status fall a full four percentage points before a partial recovery before month-end. This drives home the point that plan sponsors need to monitor funded status regularly, ideally on a daily basis."

Some plan sponsors might prefer not to have the problem at all. The potential for sponsors to stop offering pensions altogether has been a concern of Julio Portalatin, Mercer's president and CEO, who renewed his call for Congress to pass the proposed Pension and Budget Integrity Act in an op-ed in the July 5 issue of The Wall Street Journal.

Portalatin has already spoken out more than once on his conviction that the way annual premiums paid to the Pension Benefit Guaranty Corp. are counted must be changed, and that the premiums themselves must be reduced. In his latest writing, he said that the premiums "are pulling important resources from other key business priorities and undermining employers' desire to maintain pension plans … This could lead to the unintended consequence of shrinking the PBGC's premium base and thus weakening the PBGC's overall financial position as an insurance solution retirees can rely on in a worst-case scenario for their pension plans."