A student investigates the gas-phase decomposition reaction represented below:

\[ \text{XY}(g) \rightleftharpoons \text{X}(g) + \text{Y}(g) \quad K_c = 4.0 \times 10^{-5} \]

The student performs two separate trials in rigid containers at the same temperature, each starting with pure \(\text{XY}(g)\):
- Trial 1: \([\text{XY}]_0 = 0.40 \text{ M}\)
- Trial 2: \([\text{XY}]_0 = 1.0 \times 10^{-4} \text{ M}\)

To calculate \([\text{X}]_{\text{eq}}\) without solving a quadratic equation, the student assumes that \([\text{XY}]_{\text{eq}} \approx [\text{XY}]_0\). Which of the following correctly evaluates the validity of this approximation for Trial 2 and the direction of error in the calculated value of \([\text{X}]_{\text{eq}}\)?