Take this scenario:
a tool has a specific feature that could be regarded as an invaluable benefit for. However, this feature, if used in opposite direction may be counterproductive. In other words, the strength point may be the weakness point.
For example, feature X of a security mechanism could be leveraged to break that if used maliciously. So, what's the word/phrase to describe this feature? I think it should be something other than Achilles heel.