firstLineIndent can’t be set to zero
Create a text object, type some text, keep it selected and run the script:
activeDocument.textFrames[0].paragraphs[0].paragraphAttributes.firstLineIndent = 20;
alert(activeDocument.textFrames[0].paragraphs[0].paragraphAttributes.firstLineIndent);
activeDocument.textFrames[0].paragraphs[0].paragraphAttributes.firstLineIndent = 0;
alert(activeDocument.textFrames[0].paragraphs[0].paragraphAttributes.firstLineIndent);
You have two alerts saying 20, while the second one should show and actually set the first line indent value to 0 — and it does not.

-
Sergey Osokin commented
Transforming the text frame before changing the indent property helps to fix the bug. But tricks don't make writing code any easier. The same trick helps to fix ".textRange.justification = Justification.LEFT;" bug.
-
Sergey Osokin commented
Example