Idk why this has so many down votes. It's a good question, a feature that would be super nice, and you have an undertale-themed name; what's not to like?

The short answer is: no, unfortunately there isn't. Or not to my knowledge at least.

Atcoder has a button to download the testdata

Commenting for better reach since this is a much needed question. Hope you get what you were looking for.

Hey! There is one method but sometimes it's very tedious.
Analyze the test case. And find some things that are not in the previous test cases.

if(n == 1000 and a[5] == 7 and a[0] == 0){
    // print the data or do some operation with it here.
}

Sometimes it happens that there are many tests in one test case. In that case, you can check for the test case number and print the input.

int t;
cin >> t;
for(int i = 1; i <= t; i++){
    // input the data
    if(t == some_test_case){
         // print it
    }
}

No, but there should be. I really cannot think of a reason to justify making a test case data partially accessible. When it is not hidden, it should be completely visible.

well downloading the big test cases is like making a big test case, because if you download it, how do you want to debug it!? so if you want to find a test case and debug your solution, you can write another program which you are completely sure that is working (even if its complexity is too bad) then make a big test case(you can use random) and then check your solution with the other one. then you can find a smaller test which your program is giving WA. it works most of the time.

Having a feature like CSES would be nice. You can press the eye and the whole test would pop up.

It would be very useful to have this feature.

Although some people say it's impossible to debug the program with an array of N=10,000 elements, I still find it useful, and I can provide an example.

Recently, I have struggled with the problem which assumes a lot of if-branches in the solution. The structure of my solution was like this:

if (...)
    return "NO"
if (...)
    return "NO"
if (...)
    return "NO"
return "YES"

When I checked out my submission status, I could see that my program returned NO when there should be YES.

Of course, I wouldn't analyze the entire test case of size 10,000, but at least I could find out what branch led to NO answer.

To that end, I had to manually extract the test case. I did 20 submissions in order to print the input data and concatenate it afterwards.

Once I merged everything, I was able to run the test and find out the wrong branch. It was extremely helpful.

I wish codeforces had an opportunity to download tests in order to avoid "manual extraction and concatenation" of test data through 20+ submissions.