FBBIK samples the pose of the character when it initiates (normally at Start) to find out which way to bend the limbs. If the limbs are completely expanded (perfect T-pose) or inverted, then FBBIK is not able to get a good sample. Please take a look at this tutorial video for the process of fixing this problem.
If you were adding FBBIK in runtime, you can use ik.solver.SetLimbOrientations(); to make sure the limbs will bent right. Please see the User Manual page on FBBIK for more info about that.
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