Michael A. Taylor hit an inside-the-park Grand Slam on Friday, September 8, 2017. How rare is this event compared to other rare events in baseball?
I can't find stats on this rare event but the last inside-the-park grand slam happened in September 2015. The last one before that was in 1999. So it is a pretty rare event. An inside the park grand slam is usually caused by an outfielder making a mistake of some sort that is not considered an error. As a very unusual note, the outfielder involved in the 2015 homer was none other than Michael Taylor. Yep, the same Michael Taylor who hit one recently.
As a side note, in 1956 Roberto Clemente actually did even better. He hit an inside the park grand slam walk off homer.