Realistically there have probably been quite a few different times where Tyler Montain could have performed better. Anywhere from blowing coverage, missing a tackle, failing to seal the edge, etc. There are a lot of different ways it can and has happened. A specific time, though? I can not think of one without spending time going back through games to find one as I can not think of anything off the top of my head. There are plenty of situations where Montain has likely missed a tackle that ended up going all the way as well as likely many times where he failed to defend a pass that went for big yardage. With Montain, there are even other options. He spent a season returning kick offs and may have failed to gain enough yards on a drive for a successful outcome or he could have even fumbled the ball on one of them. I do not remember specifically, but it very well could have happened.
Thank you to @enigmatic, @Kyamprac, @mithrandir, @Mooty99, and @Number 82 for some of the sigs in this rotator.