My understanding is that the IRS takes the position that the compensation limit in Code section 401(a)(17) does not require that salary deferral contributions be stopped when a participant’s compensation hits the limit for the year. The clearest formal articulation of the basis for this position appears in the preamble to the final 415 regulations that were issued in 2007 [TD 9319]: "As noted above, the final regulations provide that a plan cannot take into account compensation in excess of the section 401(a)(17) limit. In addition, the final regulations provide that elective deferrals can only be made from compensation as defined in section 415©(3). However, in applying these two rules, a plan is not required to determine a participant's compensation on the basis of the earliest payments of compensation during a year."
The IRS illustrates how this applies on the following page: http://www.irs.gov/Retirement-Plans/401k-Plans-Deferrals-and-matching-when-compensation-exceeds-the-annual-limit
And in the following IRS newsletters from 2009 and 2012: http://www.irs.gov/pub/irs-tege/fall09.pdf (see the top of page 4) http://www.irs.gov/pub/irs-tege/epn_2012_1.pdf (See We’re Glad You Asked #2 on page 7)
Those IRS pieces acknowledge that a plan’s terms can provide otherwise – e.g., that “a plan can specifically require that salary deferrals cease once a participant’s compensation reaches the annual limit. If your plan specifies that salary deferrals be based on a participant’s first $260,000 of compensation, then you must stop allowing” deferrals once an individual’s year-to-date compensation reaches the 401(a)(17) limit (emphasis in original).