Match before eligible

If an employee received a match before they were eligible in the year prior (so before they met the 1 year requirement), can this be corrected?  I'm not seeing anywhere details on this type of correction. Can it be put into a "suspense account" still even though it's crossed calendar years?

There are 2 ways to correct this under Rev. Proc. 2019-19:

1. Treat it as an excess allocation. Move the amount (adjusted for earnings) to an unallocated account (aka suspense account) which must be used to reduce future employer contributions. The employer may not make any further contributions to the plan, other than elective deferrals, while money remains in the unallocated account.
2. Retroactively adopt an amendment making the employee eligible as of the date they began receiving contributions. This is only available if the employee is NHCE.

It doesn't matter if the failure crossed more than one calendar year or plan year, as long as it is not beyond the end of the second plan year after the year in which the failure occurred.

CB - while the 2-year period you refer to is required for a Plan Document Failure under SCP, I think this "insignificant error" is eligible for correction under SCP without the restriction of the 2-year period? See RP 2019-19, Sections 2.02 and 4.05(2)(a), as well as Appendix B.

If it's within the 2 year period then an operational failure can be self-corrected regardless of whether it's significant or insignificant. An insignificant operational failure can be self-corrected at any time. I don't know if the failure is significant or not based on the info presented.

It's not significant. Is this considered a mistake of fact refund?

That is not a correction described in EPCRS.

Mistake of fact is generally reserved for minor arithmetic or typographical errors. In this case the arithmetic was done correctly in calculating the match; the error was in applying the plan document's provisions.