We've all heard it, frequently from the guy who is carrying three guns, 90 rounds of ammo, two tactical folding knives, a cell phone, a couple of flashlights, a can of Mace, an expanding baton, a Kubotan, and so on. "Anything that can go wrong will go wrong." A close companion is "Better to have it and not need it than need it and not have it." While technically true I suggest that this is a poor way to plan, and thus becomes a myth that the thinking gunfighter needs to recognize.
Yes, anything that can go wrong will go wrong...given enough time, enough incidents, and so on. The actual problem is not will it go wrong, but how likely is it to go wrong at any particular time or place. The same is true of the concept that it is better to have it and not need it. Technically true, but "what is the actual likelihood of needing it and how much does having it impact your resources" is the better way to look at the issue.
Let's look at the issue from a few different viewpoints. A question I often see is "how much spare ammunition should I carry" or the similar "how many spare magazines should I carry?" At which time we begin hearing horror stories about being attacked by roving gangs of street thugs, or shooting someone and then having their friends come to their defense and having an on-going, running gunfight, or some such. Folks, it is possible, but not too darned likely. You are going to solve the problem with what you have in your gun, or the chances of you solving it at all are almost non-existent. Even in law enforcement, with a much more offensive role, reloads are needed in a very small percentage of gunfights.
Let's say that you have 30 rounds with you. Do you really think that that you are going to be able to accurately and effectively fire 30 rounds at the bad guys? Equally important, if you were attacked by a dozen people, (A)do you really think they would press an attack after you shot the first 3, or 4, or 5, etc. and (B) if they pressed the attack do you really think you would be able to shoot them all before they got you? Let's think about it.
"But wait!" comes the cry. "What about if I have a malfunction and need to clear it? That requires another magazine." First, if you are worried about your weapon malfunctioning, you need to get a different weapon. Yes, I know that guns do malfunction. I see it on the range quite regularly. Why do they malfunction? Bad ammo, cheap aftermarket equipment, modifications designed for the range instead of real life, and so on. A well made, quality firearm that has been properly checked out and is well maintained, using ammunition proven to feed in that gun will not suddenly decide to start malfunctioning on you. Yes, magazines are the weak link in most autoloaders, but not good magazines. If you are going to the range and practicing and checking out your equipment, you will know what works and what doesn't. If a magazine malfunctions don't carry it for serious social purposes. Use that magazine only for the range. And if it continues to cause problems, throw it over the berm and get rid of it!
Second, let's think about this for a moment (Thinking Gunfighters, remember!). A spare magazine only addresses problems that are related to the magazine. If one wants to worry about malfunctions one should look at all malfunctions. After all, isn't that the essence of Murphy's Law? Why worry about and provide a solution to one narrow malfunction problem. Let's solve ALL of our malfunction problems and just carry a spare gun. Realistically, what are the chances that your quality-made, well-maintained firearm will pick this particular time to have a malfunction? Given that, what are the chances that particular malfunction will be the result of a magazine that is defective? If we truly believe in the Murphy concept carrying a spare magazine is rather silly.
OK then, but still isn't it better to have it and not need it than the other way around? Again, sure, but let's think about it. We can't carry around everything, so we need to rationally consider what equipment will be useful to us and what won't. It is always going to be a compromise. Most of us don't carry around a big-game rifle to kill a tiger if it attacks us. Why not? Isn't it better to have it and not need it than need it and not have it? "Wait a minute" you say. "I don't need that rifle because the chances of me getting attacked by a tiger are so small that I don't need to worry about it!" And with that you have effectively done away with the myth of "better to have it and not need it." Having something does cost us. Whether it be money, time, physical effort, convenience, or any of our other resources, there is a cost. We should balance that cost against the need.
The reality is that we all make decisions about what we will need, and we all compromise what we carry based on what we think we will need. Based on perceived need one person might decide a 1911 with 8 rounds meets their needs. Another person might decide that a Glock 17 with a spare mag and 34 rounds meets their needs. But the decision on where to compromise on what to carry should be made based on a reasonable and realistic understanding of what is needed (risk assessment) accompanied by a cost/benefit analysis.
"Remember Murphy's Law" and "better to have it and not need it" are the rationalizations of people who are unable to decide what their needs actually are or people who are unwilling to accept the conflict between the world of fantasy and reality. No matter what the case we will all compromise on what we decide to do for our personal safety and security, even though we might be unwilling to acknowledge that fact. The important thing is if we determine that compromise based on thinking about it, or do we base it on the non-thinking mythology of Murphy's Law?