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Bug
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    Resolution: Duplicate
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P2: Important
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    None
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    5.6.1, 5.7.0
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    None
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    Windows8.1
official qt-opensource-windows-x86-msvc2015_64-5.6.1-1 installer 
running the following JavaScript code in the script example application (Qt\Qt5.6.1\Examples\Qt-5.6\script\qscript) produces different results for debug and release mode:
var str = "teststring";
str.lastIndexOf("i");
in debug mode it works correctly and returns '7', while in release mode '-1' is being returned in all cases.