Can only the 100Mbs part of a 10/100Mbs router fail?

[Drew Van Zandt]

If the crystal on the router that feeds the 10/100 PHYs drifts out of
spec, 100 would stop working, 10 would probably be more forgiving.  Is
this likely?  No, but it is possible. Just my $0.0199999999999

--DTVZ