Summing Up Problem 116 - DescriptionInstructions are not clear

Yeah mate, this problem is intended to be solved only on Brainfuck (with Python it would be too easy, don't you think?).

See, your code have the following line:

arr.append(arr[0]+arr[1])

Brainfuck interprets this line as follows:

1. Command + orders interpreter to increment value in current memory cell - after this value of current memory cell becomes equal to 1.
2. Characters 'a', 'r', 'r' ignored.
3. Command [ signals the start of loop.
4. Character '1' ignored.
5. Command ] check the loop end condition. Because the value of current cell is 1 and is not equal to 0, program returns to previous command [ and loop continues.
6. Command ] reached again, loop continues...
7. And so on, until limit of operations exceeded - thanks god such limit exists.

Please refer to article Brainfuck Wiki for details.

Additionally, I took a look at problem statement / instructions and I think they are pretty clear.

You are asked to modify a Brainfuck program - obviously result should be Brainfuck program too!