I had a very strange experience on a flight from HPN (White Plains, NY)
to BID (Block Island, RI) this morning. We were in an Arrow equipped
with:

GPS: Apollo GX-50
Nav/Com 1: Apollo SL-30 Nav/Com (slaved to the GX-50)
Nav/Com 2: King KX-155 Nav/Com

plus some other stuff whose exact model numbers I can't remember (King
DME, Narco xponder, some random audio panel).

We were on V3 between Carmel and Hartford, level at 7000, navigating by
GPS. We tuned Madison VOR (110.4) on the SL-30 to confirm cross-radial
fixes. Instead of receiving MAD, we got HVN (a nearby VOR). The morse
ident was ..../...-/-., both by ear and by the auto-decode of HVN on the
SL-30. We had 110.4 in both the active and standby slots of the SL-30.
HVN is 109.8.

Then we tried putting 110.4 in the KX-155, and got the same thing! I'd
admit I didn't actually try to use the nav signal (i.e. figure out what
radial we were on), but the morse ident was unquestionably HVN.

We cross-checked the two en-route charts we had, and a sectional, so I'm
sure we had the frequency correct. I just looked them up on airnav when
I got home to confirm as well.

I can think of no possible hardware fault or mis-cabling that could
possibly cause those symptoms. Given that there were two of us, and we
cross-checked the freq on 3 different charts, operator error seems
unlikely (but never something to be ruled out). Can anybody think of a
rational explanation?