Well, the only time I went down on a group ride was when the leader slowed without signaling. My husband was behind him, thinking this guy knew how to paceline. After he hit, he knew he was going down, and swung wide so I wouldn't hit him. Of course, my eyes followed him, and I went down, along with the 2 behind me. We were going around 20, which may not be fast for some, but for this group it was. I wasn't hurt, but I was shook up, my helmet scraped, and I had a headache for a week. We left the group and rode to the emergency room after my husband's elbow swelled like crazy (it was just from the medication he takes).
Not my best ride and I will never go on a ride with that leader again. He's very nice, but doesn't know how to lead.