title: Mathematica evaluates expression once, then returns the expression unevaluated
body: <p>Mathematica 8.0.1 on Mac OS X 10.7.2 does this: Quit the kernel and execute</p>
  
  <pre><code>MathieuS[MathieuCharacteristicA[  1, -(1/4)], -0.25`, 15.707963267948966`]
  MathieuS[MathieuCharacteristicA[  1, -(1/4)], -0.25`, 15.707963267948966`]
  (*
  5.10119 10^-15
  MathieuS[MathieuCharacteristicA[1, -(1/4)], -0.25, 15.708]
  *)
  </code></pre>
  
  <p>That is, the first time it numerically evaluates the expression, while the second time it returns the unevaluated form. </p>
  
  <p>Why? Or have I just spent too long staring at my screen and am doing something stupid?</p>