Nyquist rate determination and oversampling
you actually have to sample at more than twice the highest frequency.  if $B$ is the bandwidth or highest frequency and $f_\text{s}$ is the sampling frequency, then to satisfy the Sampling Theorem (for normal baseband sampling):

$$ 2B &lt; f_\text{s} $$

it is not

$$ 2B \le f_\text{s} $$

sampling at exactly twice the frequency of a sinusoid is "critically sampling" and loses amplitude or phase.  it can't get both.  in your case, it is sampling the sinusoid when it crosses zero.