A concrete example: say Beauty(R,D) is trivially decidable (say it returns (popcount(R)+popcount(D))%2. Your argument "proves" this property is undecidable, but it's definitely decidable. So some step in your argument is wrong.
Another concrete example: Assume the halting problem is decidable. I would like a cookie. But the halting problem is not decidable. Therefore I would not like a cookie.
"I confess I am a frustrated academic"
in English vernacular being a frustrated X (as in a frustrated academic, frustrated poet, etc.) means that the person is actually not that, but have in some way been prevented from being the thing (frustrated) and have perhaps rancor in regards to the frustration and towards those who actually are the thing (this second feature being sometimes implied depending on who is doing the description of the person as frustrated)
on edit: formatting, clarification
I was only dimly aware of the second meaning of the word so I'm not sure how you would interpret it if you had a good understanding of the multiple interpretations.
How to prove your pizza is poisonous.
Let's put some cyanide in your pizza. Now let's assume by contradiction your pizza is not poisonous, but alas, in contains cyanide which is poisonous then so it is the pizza.
Some particular pizza might very well be non-poisonous, but this is a statement about all pizzas.
RECIPE TO PROVE ANYTHING IS INCOMPUTABLE (According to this paper).
Example, beauty is incomputable
Step 1. Assume there is an algorithm Beauty(R,D) that given the program R and the input D, will return True if it is beautiful and False otherwise.
Step 2. Create a high-order function that is a simple wrapper of the halting problem over the algorithm created in step 1
Theorem. Beauty is undecidable.
Proof. Assume by contradiction that the halting problem is decidable, then, feed it the function created in step 2, thus you can always have an answer for step 1, but since the halting problem is undecidable this is not possible.
For more variety please substitute beauty for "awesomeness", "arachnid power", "goodness","fear" and so on.