In cells formatted for numbers, fractions are converted to dates. I feel this conversion should only be made in cells formatted for dates, not numbers in general.
Cells are not "formatted for numbers" or "formatted for dates" input. Cells may have a *display format* assigned that is not an input mask. If your locale happens to use the '/' slash date separator and you don't want a simple fraction to be recognized as abbreviated date input then remove the D/M or M/D (depending on your locale) pattern from the Date Acceptance Patterns under Tools -> Options -> Languages Settings -> Languages. Or precede the fraction with a 0 to force a fraction input, like 0 1/2