Generally, we only offer a קרבן חטאת, sin offering, for violating a serious transgression, inadvertently, or בשוגג. Serious is defined as a sin that if committed intentionally, במזיד, and with witnesses, the transgressors would get death by the court or כרת, being cut off from the Jewish people.
This is why violating Shabbat בשוגג, requires one to offer a קרבן חטאת. Intentionally violating Shabbat is סקילה, death by stoning.
There are four exceptions as to when a חטאת is offered even though the penalty is not so severe. The four cases are: one who has relations with a שפחה כנענית, non-Jewish maidservant, a נזיר who accidentally becomes טמא, a false oath while testifying, and a false oath regarding denial of possessing something deposited with him. The latter two cases are called, שבועת העדות and שבועת הפקדון.
We see from all of this how severe it is to misuse speech. We must be careful to be truthful with all that comes out of our mouths.