It's easy to damage the internal threading that the pedals thread into, and relatively difficult to manufacture the cranks properly to where this error doesn't exist. I've had better luck with steel cranks that don't fatigue, than with aluminum ones that do. Using steel pedals with an aluminum crank is probably greatly increasing the risk of such a failure occurring.
Not only has this issue occurred on my custom-built electric velomobile, but it has also occurred in my Milan SL velomobile. My custom build, I designed with a removable nose piece, which allowed easy access to the cranks. But the Milan has no easy way to access the crank, and to get the pedal to thread back into the crank took about 2 hours of struggling with the bike laying on its side and bending at odd angles in a gas station parking lot in 20F degree weather before it would cooperate and thread back in. I had to clean aluminum shavings out of the crank. It was not fun.
I always grease the threads inside before installing the pedals in order to help prevent this, but it only does so much.
Luckily, since I use recumbent trikes with bodies on them, if the pedal falls off, I'm not going to get injured because there's a floor.
That Milan SL is a real pain in the ass to work on though. A rear wheel innertube replacement is a 2 hour ordeal. On a normal bike, I can do it in under 10 minutes. This is why my custom build is getting solar car tires. Not only do I not want to have to worry about it, but at the speeds I ride(car speeds, not bicycle speeds), a rear wheel blowout could cause me to lose control and veer into a pole or even oncoming traffic. Plus the custom build is going to soon have enough power to accelerate faster than most cars and top 100 mph, and bicycle parts are NOT up to the task. I'm using a mix of ebike parts, motorcycle parts, and ATV parts, with a bicycle drivetrain for pedaling. I'll be able to use all the same tools to service it should something go wrong.
Legally, with both vehicles, state highways are fair game, and I sometimes cruise 45 mph in the custom build on state highways, but I won't be able to use the interstate highways even after I make the upgrades to allow 70 mph cruising, because legally, it's still a "bicycle". With the Milan, I have to be very picky where I ride it because of ground clearance issues and the fact that it has no motor, making very steep uphill climbs a walking-pace slog(NOT good when the automobile traffic is doing 45 mph), but the custom build having a motor makes hills no problem at all. The reactions from police, which frequently pull me over in both vehicles, are mixed.
In the long term, transportation on a per mile basis doesn't get any cheaper or simpler than trying to build cars out of bicycles. The components that compose my bikes are mostly protected from the elements, and I can get over 15,000 miles out of a chain before it is stretched to 0.75%. As mentioned earlier, cables are my most frequent replacement item. Per mile, tires are the largest component of recurring maintenance, at $0.01/mile, followed by chain at $0.004/mile. The custom build uses so little electricity that is is one of the cheapest components of its operating cost, as I get 150-200 miles on 1.5 kWh. The worst aspects are getting messed with by cops and every random idiot with a phone recording me. Just be aware of the local laws, and whenever observed by police, maintain the appearance of following them, and one should do okay operating such a vehicle. When I take the custom build into Illinois, I have to limit it to 750W/28 mph to be "street legal", but when I'm out in the country and no one is looking, I turn the limiter off and cruise at 45 mph. The Milan, being unmotorized, I go as fast as I can pedal it, but without a motor, it is slower up steep hills than a normal bicycle would be, even though I can reach 50 mph on flat ground in it during a full effort sprint, which takes about 2 miles of uninterrupted hard pedaling, there are hills where I do 3 mph climbing for the same effort it takes to maintain 30 mph on flat ground.
Both vehicles have space for my tool bag, food, water, intoxicants, and camping gear. Both vehicles can also fit through a doorway, perfect for camping in abandoned buildings, or in the case of the custom build, hiding it from police if it ever gets involved in a chase.
Almost this same thing happened to me. My failure was at other end of the crank. I was still a dumb kid at the time so I naturaly just kept rideing it as it got loser and loser. Then one day while I was rideing down a hill the whole right crank arm just fell off. And then I very nearly ran off the road. The threading in the pedals and bottom bracket can come lose over time. There not supposed to but it happens. Checking the bottom bracket and pedals for play fairly regularly can prevent this. Tightening lose components will keep it from shaking around which will damage the threading and destroy it. That being said. Not all components work. Some cranks will just never hold a pedal. Even from good manufacturers. Even right out of the box. Ive opened at least two sets of cranks, one shimano and one campi I think, that just didn't work and had to be sent back.
