BMO wants to hear that you don't hide or ignore your mistakes. They are a company that knows how to improve and grow based on learning from past failures. How do you think they have been so successful?! Of course, it's difficult to come up with a good example, but you can do it if you plan! This question is where interview preparation is essential!
Just remember, the interviewer isn't worried about whether you will fail again. They're not looking for faults. The interviewer just wants to find out how you will handle failure since it is sometimes unavoidable! Think about a situation that occurred, at work, school or when volunteering that you regret. What do you wish you could have done differently? Think about what you learned.
Another way to think about this question is to reflect on your most significant learning experience. Sometimes lousy planning can get us into emergency situations, like not looking at a map and getting lost in the middle of nowhere, or not studying for a test that we thought would be much easier than expected! Keep it simple, and focus on how you grew from the experience!
"I failed a statistics exam in my first year of University. I was embarrassed and felt like a huge failure. However, rather than give up, I hired a tutor and asked for a re-do of the exam. Luckily my professor agreed, and I passed with a very respectable grade. Sometimes, when it doesn't come naturally, we need to take some extra steps and try harder than normal to succeed."