Question & Answer

                                  Is it possible to accidentally commit a mortal sin?

                                  No.  For a sin to be considered mortal, three conditions must be met:  "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent" (Catechism of the Catholic Church (CCC), #1857).
                                 Grave matter means that it must directly conflict with the Ten Commandments.  While all forms of sin are bad, the severity of the misdeeds committed, as well as how strongly they harm ourselves and others, decide whether the sin is mortal or not.
                                 If the offense being committed is indeed grave, the perpretrator must be fully aware that the actions involved are sinful.  Finally, knowing that the action is grave, the person must willingly choose to commit the sin of his or her own free will.  These last two conditions make it impossible to sin by accident.
                                 By using this knowledge, we can avoid actions that separate us from God.