The T -test is probably the most popular statistical test; it is routinely recommended by the textbooks.The applicability of the test relies upon the validity of normal or Student's approximation to the distribution of Student's statistic t n . However, the latter assumption is not valid as often as assumed.We show that normal or Student's approximation to L(t n ) does not hold uniformly even in the class P n of samples from zero-mean unit-variance bounded distributions. We present lower bounds to the corresponding error.The fact that a non-parametric test is not applicable uniformly to samples from the class P n seems to be established for the first time. It means the T -test can be misleading, and should not be recommended in its present form.We suggest a generalisation of the test that allows for variability of possible limiting/approximating distributions to L(t n ).