Consider the Flip-flop. Flip-flops of various forms have been worn for centuries, with their first depictions engraved in ancient Egyptian ruins. They have survived in one form of another since then, humanities constant associate, older than Christianity, older even than writing. The Flip-flop endures.
But they are not our friends.
In 1998, a spate of injuries were observed to occur in young girls in Newcastle. The children reported similar symptoms, with damage to their crotch areas, with bruising and occasional bleeding. They were immediately referred to Lindisfarne hospital, to determine whether they were being sexually abused. Fortunately this was not the case. The problem was in the footwear they were using.
At the time of the accidents, they wore sandals. Often , the heel of these sandals would extend beyond the actual heel of the shoe. Exacerbating the situation was that since these were children, often they would be wearing shoes slightly too big for them, in the hope that they would eventually "grow into them". This made them unstable for the children to walk on. These factors all contributed to this spate of accidents.
In this scientific paper, they describe five cases where young children slip and fall. One foot buckles, bringing the hard heel of the flip flop up to the perineum, striking it with some force.
This kind of injury, the straddle injury, is a common form of injury, and although painful, is nothing to worry about.
Either way, I'm avoiding Flip-flops.
de San Lazaro C. & Sivaramakrishnan S. Summer sandal genital trauma.1998, Journal of clinical forensic medicine, PMID: 15335556