If you enter (-1)^(2/3) into Speedcrunch, it gives a result of -1, if complex numbers are disabled. If complex numbers are enabled it gives a correct result of -0.5+0.86602540378443864676j.
There are three valid answers: 1, -0.5+0.86602540378443864676j, and -0.5-0.86602540378443864676j. The value of 1 should be considered valid even if complex numbers are disabled: You can evaluate (-1)^(2/3) as either ((-1)^2)^(1/3) or as ((-1)^(1/3))^2, both of which can be completed without complex numbers, yielding a result of 1 in each case.
To my knowledge there's no way to get -1 as a valid result here.