No worries DY. I write quicky and often my writing only resembles English (which I beleive is ok according to the rules)
there is a general rule that we never do anyting to be mezalzel yom tov. In other words Every yom tov sheni is Safek yom tov rishon, safek chol. Mideroisa there is no second day of yom tov sukkos it is either the first day or the first day of chol hamoed. So while on the third day of sukkos we say indicate in our davening and laining that it is either the second day or third day of sukkos (both chol) we never do this on Yom tov. Even tefilin which is De’oraisa even those who wear on chol hamoed dont wear it on yom tov sheini even though it is safek chol (should say safek deoraisa lechumra) we arent mezalzel yom tov. Not with words and certainly not with actions.
This is why we dont shake arba minim on shemini atzeres doing so would clearly indicate that we are mesupek that it is really the 7th day of sukkos (yom chol) and we dont want to be mezalzel yom tov (Muktza is a side issue and is circular since if we took lulav on shemini atzeres it wouldnt be muktza, so not taking it solely BECAUSE it is muktzah is circular since is only muktzah since we dont use it).
The Gemara of course says we sit in sukah on shemini atzere. Many rishonim including Tosfos on the spot explain that since there are times that a person eats in sukah (nice weather) it isnt obvious that he is announcing today is chol so there is no zilzul. I beleive tur and Rosh say this too.
It seems obvious and I believe the Orach Hashulachan (!!!) says this beferish that if it is obvious that eating in sukah is BECAUSE maybe it is 7th day of sukkos it shouldnt be done(I dont recall if he says this lehalacha or just a limud zechus I have to double check it seems surprising that he would say not to eat in sukah).
Now I have to tell you that I have never eaten in a sukah unless it was Sukkos. Even outdoors depending Isnt something I have done or seen many poeple doing in the New york area in October. Eating in the sukah on Shmini atzeres is clearly becasue maybe it is 7th day of sukkos. This is a zilzul to yom tov and shouldnt be done.
Of course I am sure those who do eat in sukah have a good reason too.
(an exception to this is counting sefira on he second day of PEsach therby signaling that perhaps it is chol. The nosei keilim talk about this.
The Erev tavshilin al tnai MIGHT be another example. Seems debatable
I cant think of any other example)