In Germany, in general...
You are expected to write down the selling machine's number or take a photo with your cell phone. This is in case you are being controlled, so you will only buy a ticket for 2-3 Euros rather than being fined 60 Euros. You are however also expected to actively contact personnel and tell them about it, and if there is none, you are to exit on the next station and buy your ticket there (though you will probably get away if you can just provide the broken machine number in case you're being controlled).
You are kindly asked (but not required) to help DB by calling their 0800 number and informing them. The number is found on every selling machine.
I was born in Munich and my grandmother still lives there, I go to Munich regularly to see her. Municians, in particular MVV personnel have a reputation of being slightly less than friendly towards foreigners (which includes non-Bavarian Germans). I can personally not confirm this. I cannot even say anything bad about Munichian bus drivers who have an extra bad reputation about being unkind ruffians.
The MVV ticket system is sheer horror. Single fares are so expensive that they almost make you faint, so most people who need more than one fare (or who are in a small group) use the Streifenkarte (costs 12.50€ if I recall correctly, might be 13€ now). A defunct selling machine would therefore usually be not such an issue since you already have your ticket in your pocket.
This kind of ticket is cheaper and very flexible, it can be used by a single person for several trips over one or several zones, or by multiple people for one or multiple trips.
Now, buying the Streifenkarte is already a bit of a challenge to a foreigner, but using it correctly is yet another dimension of joy. It involves counting the number of zones, or hops, or both. And sure enough, the number of hops allowed for a short-trip (1 tick) is different using the Tram or the U-Bahn.
Be sure you have someone show you, and be sure to look on the plan every time. And pay attention which way around you put it into the validator stamp.
The system of folding the Streifenkarte correctly is so complicated that unless you have some practice and pay much attention, you are likely to do it wrong (either having no valid ticket, or paying a full trip when a short trip would suffice, or invalidating the whole thing). My father, who is (obviously) a couple of years older than me has accidentially invalidated the whole ticket more than once.
On the other hand side... I have never been controlled in Munich. Never, not once. So, the risk of being caught with a wrongly stamped ticket seems to be rather low.
If you are planning a trip which involves more than 3-4 fares within Munich, or more than a single person, or pretty much every trip outside the city limits, then the Bayern Ticket may be a valid option, and quite possibly the best. You can only get it at selling bureaus (DB, but the MVV bureaus sell it as well).
This ticket costs you 25 Euros (used to be 15€ ?!) for the first person, plus 6€ for every additional person up to 5 people. It's valid for 24 hours, and you can go everywhere you want, not just within Munich but within Bavaria, any number of trips.