When playing as Mira, I’ve had numerous times where a button combination aside from MP + MK will cause a Counter Breaker attempt mid combo (sometimes she will jump in the air performing the Counter Breaker stance).
The frequency seemed to drop after the anniversary edition, but it is still happening.
Already documented?
I rarely play as other characters so I couldn’t tell you if this happens outside of Mira.
Killer Instinct version: Windows Edition
Windows 10