I would believe this if the IRS actually had a system for tracking W-2 forms and tracking the cumulative retirement contributions made for all W-2s in a given year.  Since you are not required to provide that information on a tax return, I very much doubt the IRS is tracking this.  And I am not worried about the IRS coming after a client twenty years after the fact for a $183 overage.

In a separate case, I recently had clients who had been making ineligible Roth contributions for the past 17 years -- they had no idea there was an AGI ceiling.  They voluntarily wanted to make things right after learning about this rule during a workplace retirement seminar.  Did the IRS catch this?  Nope.