Neither of them mean "by accident". They can be completely intentional. Occasional: Occurring, appearing, or done infrequently and irregularly. ( https://en.oxforddictionaries.com/definition/occasional ) Sporadic: Occurring at irregular intervals or only in a few places; scattered or isolated. ( https://en.oxforddictionaries.com/definition/sporadic ) Both occur irregularly, but "sporadic" events are scattered around either time or location.

They don't mean by accident: Occasionally means 'not so often' or 'from time to time' Sporadically means not very often at all and in an unpredictable pattern