Canadian 1968 10 Cents / 25 Cents 80% Silver Transition Error? Any Known Coin Examples?

"Ping" test proves nothing, it's just an educated guess.
In the 21st century we have XRF, and that's what it will take to find what you are looking for.
If you can afford it then go for it.
Let us know if you find one.

Well, I don't know what you mean by "XRF", but good hearing skills will
be able to make the comparisons, and set the suspect coin in the right
category when compared with a known 50% and a known 80%. You don't
need a stupid cell phone to do that!
The only thing better than ping test would be a gravity test that requires letting
the coin drop in water around a rope, and weighing (with the tare for the tools).
I never did that, but if the ping test proves to be a 1968 at 80% silver, then it's
time to do the gravity test to confirm without a doubt...

You should step into the 21st century.

XRF stands for X-ray Fluorescence, a spectrometer that measures composition. I suppose an 80% planchet could have been "stuck in the hopper" like the US 1943 bronze cents.

You're right that not *all* 80% silver coins ring the same. I have a 1960 dime
that rings really dull, almost no silvery ringing when spinning on the table, but
if you listen carefully, you hear a "base tone" that sounds like all other 80%
coins I have tested in my life. Some other 80% dimes ring less than others, but
they all have the base tone. As of 50% coins, all the ones I have had so far ring
the same, but have a different base tone, proving they're 50%. I guess I have not come
across yet a 50% that rings dull, but it most probably exists. I don't know why that, but
the ringers at the Mint would certainly have rejected the 1960 if they came across it, but that doesn't mean it's a fake. The XRF test (no I know what it means, and they use that in forensic science to analyze evidence and find criminals) is another test to confirm sound test, but it doesn't mean sound test is not good.

Same for US coins. Especially half dollars. Sound test is obvious to classify pre-1965 90% silver, 1965-70 40% silver clad, and 1971+ nickel clad. ALL 3 have a different ringing sound, even the modern clad has no silver at all. All of their coins apart zinc pennies and nickels have "silvery" ringing even modern clad. I'm sure this is because of the copper contents. BTW, none of them is magnetic. So copper and silver alloys have a different base tone according of the composition of each. In Canada, alas, we don't have a denomination to CRH as cool as US Halves, but somewhere in 1997, I was lucky enough to sneak in a Currency Exchange teller drawer and ask for "USA 50 cents". She pulled out 20 halves or so, and I picked up 3 "Walking Liberty" 1934, 1941 and 1943, as well as several 40% Kennedys. Later on I bought a 1963 Franklin half. I carefully "Ping tested" all these 4 90%. The 1934 and 1941 ring identical, but surprisingly the 1943 and 1963 also ring identical, but have a slightly different base tone as the 1934 and 1941! That surprised me! All 4 must be 90%, but was there a "secret" undocumented wartime slight alloy modification that remained until 1964? I'm much convinced so, and I'm sure the XRF test will pick up the difference. To continue investigating, I'd love to pick up some 1942s, other 41s and 43s as well, to know where they belong and when exactly the secret modification occured. Being able to test other years as well would allow me to prove or disprove my theory, but I have no others so far (being in Canada). Remember that in 1942, they switched nickels to a 35% silver alloy for wartime. This is documented, but did they do secret subtle changes to their halves or other 90% silver coins as well?