Any situation where you have M userspace jobs running on N system threads, i.e. the number of tasks is different to the number of system threads.
Normally this occurs because you're running a large number of "green" threads on your own scheduler which schedules onto a thread pool underneath. This is good if all your threads are small/tiny since userspace thread creation is cheaper than creating an OS thread but if your jobs are long-lived then your userspace job scheduler is really just adding additional scheduling overhead on top of the overhead that the OS already has for thread scheduling and you would have been better with OS threads. If your M:N threading requires separate stack space for each job, there can be a sizeable overhead (this is why Rust abandoned M:N threading).
If you're crossing the FFI boundary a lot, any overhead adds up quick. For example, drawing a bunch of small objects using Skia, performing lots of OpenGL draw calls, allocating LLVM IR nodes, or calling a C memory allocator…
One of the nice things about M:N is it decouples concurrency from parallelism. That is, your application can spawn as many inexpensive green threads as the task requires without worrying about optimizing for core count or capping threads to avoid overhead, etc. With Go 1.5, underlying system thread count will default to the same as the system CPU core count.
It's noticeable to the end-user only in it's negative performance implications in certain situations, making things slower than they would be otherwise on the same hardware. It's a low-level construct, it is not directly noticeable to the user either way. The negative performance implications are largely under heavy load. The post you replied to gave some more specific situations.
Normally this occurs because you're running a large number of "green" threads on your own scheduler which schedules onto a thread pool underneath. This is good if all your threads are small/tiny since userspace thread creation is cheaper than creating an OS thread but if your jobs are long-lived then your userspace job scheduler is really just adding additional scheduling overhead on top of the overhead that the OS already has for thread scheduling and you would have been better with OS threads. If your M:N threading requires separate stack space for each job, there can be a sizeable overhead (this is why Rust abandoned M:N threading).