brake calipers and missing bolts

I have a 97 Ford Ranger and, a few years ago, replaced the fromt brakes (including new rotors). As I had not done it before, I felt that I was
very careful in the work to be sure I didn't do anything wrong. All seemed fine until about 2 weeks later that I began hearing an intermittent knocking sound in the fromt right wheel. I didn't notice anything right away but decided to take it to a mechanic to get an expert assesment of the situation. On my 10 mile trip to the mechanic, the sound got VERY bad and as I was slowly making the last right turn to get to the mechanic (about 200 yards down the street), my right front tire suddenly blew (or so I thought). It was easier to have a nearby tow truck haul my vehicle the remaining 200 yeard instead of just changing the tire in the busy intersection. The problem became immediately apparent when the mechanic pulled the wheel off. The rubber of the tire was perfectly intact; however, one of the bolts holding the brake caliper had been missing which allowed the caliper to rotate outward about the other bolt and, as I drove, the caliper scored a line around the inside surface of the metal rim until it completed the cut and split my rim in half like a bagel. I was shocked and relieved that it occured at a very low speed. Also, since then, I have assumed that I must have not been careful enough when I replaced the bolt holding the caliper.
I mention this story because, just a few days ago, I was with my sister in her 00 Ford Expedition when, as we got off the freeway to get gas, we heard a horrible clunking and grinding noise from her front right wheel. She also indicated that the front end was shaking and that it was difficult to move the vehicle the remaining few feet into the gas station parking area. Upon inspection, I was able to see that her caliper was in contact with the inside surface of the rim and that a shiny line existed where the caliper had begun to cut into the rim. She had her brakes replaced by a friend about a week prior to this incident but he is a mechanic himself and, based on experience, not prone to careless mistakes.
This may be just an incredible coincidence or perhaps both my sister's mechanic friend and myself were careless in our work; however, I am now wondering if anyone else has had expereinces like this. Is it actually more common than I would think?
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