If you wear flip flops or thong style sandals, you need to pay attention to the depth of the straps. These two pair of sandals were brought in today – for the same foot, even though they are complete opposites in terms of design.
Notice how the sandal staps on the left are high and the ones on the right aren’t? Even with a casual flip-flop, you need to match the volume of the shoe to the volume of your foot. In this case, the flip-flop on the right was a perfect fit for a shallow (flat) foot, whereas the left style was much too deep.
If the shoe doesn’t fit snug, you’ll wind up gripping you toes to keep the shoe on, which may cause hammertoes and that’s not good for your toes or feet.
Dr. Jenny Sanders Shoe Blog 
Just curious – I’m a little confused … I read or heard recently that flip flops are bad for feet period because they were fantastic for causing plantar fasciitis. This was much to my dismay b/c I have a pair of Chaco flip flops that I wear everywhere the summer (they actually have arch support and some cushion so they don’t cause me pain). Is this true?