Another example of a vain oath is when he swore about something that is impossible for him to do, or which is prohibited by the Torah, and in this sense, impossible. Thus, if he knew a testimony and swore not to say it, it is a vain oath, since he is obligated by the Torah to testify if he knows a favorable testimony about someone. Similarly, it is a vain oath not to put on tefillin.
For vain oaths, one is liable to lashes if the violation is willful, but he is completely exempt if he did it by mistake.
Art: A View Of St Mark's Column, And The Doge's Palace, Venice by Edward Pritchett