Not a big deal, but had we had a Script Supervisor, this wouldn't have happened. A Script Supervisor makes sure that everything on the script is captured on set, and if anything changes in the script, those changes are noted for later. No one was looking at the script during these shots because our small crew was busy elsewhere.
MathTacular4 Script Supervision Issue
Because everyone has so many jobs on set with a small crew, things like this happen. It's not too big of a deal, unless our actor was gone for good, or wouldn't come back for less than a million bucks. Then this would be a huge deal. Thankfully our actor is "in house" and so it should be pretty easy to grab the shot we need.
Much of the editing process is figuring out how to fix mistakes and issues that are not noticed until long after filming is complete. The solution for this is to go back and get the shot again. If this was going to end up being too expensive or impossible, I would cut to a shot of one of the other actors, and just redo the audio for that bit. It wouldn't be as nice, but it would work.
Your Media Production Mentor