Shared Data and Race Conditions
A single-core MCU with interrupts enabled runs a concurrent program, and the reason people are surprised by that is that the concurrency is invisible in the source. There is one main, one call stack in view, no threads to create, and nothing that looks like it could run at the same time as anything else. But between any two machine instructions the hardware may insert an entire function of someone else's code, and if that function touches a variable you were half-way through updating, the update is lost — or worse, the value it reads is one that never existed.