It seems like everything has at least one component notorious for being the thing that breaks (first). Often, that thing is a safety switch. Sometimes, it’s something like a shear bolt that’s actually supposed to break. Sometimes it’s simply the most complex piece or the one that works the hardest.
The first category of “things likely to break” is those that are designed to protect you. The safety switch was (at least in theory) invented in the early 1900s by an engineer working in Detroit. While the purpose of a safety switch is to stop a machine running (a riding lawn mower when you aren’t on it, or a snow blower when a bar isn’t compressed), it isn’t designed to break. The safety switch exists to protect you, not the machine. In fact, it exists to protect you from the machine. A safety switch isn’t a major part of design (it has no functional purpose within the machine itself) and is often a last minute cheap plastic add-on that gets a lot of use. They are often supposed to break in ways that stop the machine working (for safety reasons). It’s unfortunate that the easiest way to fix a broken safety switch is frequently to bypass it, which rather defeats the purpose.
The second category of “things likely to break” is those designed to protect the machine. A shear bolt exists to protect the machine from you. It’s purpose is to break so that something more important doesn’t- which is sort of the purpose the tonsils seem to serve in the immune system. The shear bolt keeps the machine (and any complex, expensive, difficult to replace parts) safe from operator error, by breaking under slightly better conditions than would break other parts and stopping the entire machine when it does so.
Both a broken safety switch and a broken shear bolt should stop a machine working entirely. The third category is different, and in it things can work badly, partially or not-at-all.
The last category is the most annoying, because it’s for the things that just break. Bad design. Complexity. Under the most strain. In septic systems with lift stations, it is, almost inevitably, the lift station that malfunctions. Within that, the most complex piece, the one under the most strain, is the pump. In a computer, it’s often (at least for those of us with cats) the fan (cat hair makes them work harder). In my coffee pot? The heating element.
It’s useful to know which part is most likely to break, because it’s the first place to look when problems occur – but it would certainly be nice if they were always as cheap to replace as shear bolts!