Agreed. Absolutely
anyone can have a bad night... experience will lower the amount of times these things happen, but I don't care who you are - Greg Wilson, David Stone, the ghost of Houdini himself - you'll still have the occasional night when nothing seems to go to plan.
Also, if "add a number" routines were such a terrible, cringe-worthy idea, why do so many of them exist?
What's important BC is that you pick yourself up, learn from any mistakes you made and do what you can to avoid them happening again. In this instance (and this might not be correct, I'm second guessing from the info given) you have given 2 hints that these spectators were having a bit of a wild time, maybe had a few to drink etc... not being able to add up, not paying enough attention to the cards which might suggest they were doing it a bit half heartedly, and perhaps even dropping the cards...
What it sounds like (and again, second guessing) is it wasn't so much about spectator management as spectator or effect selection. If people are having a night on the town, a run of all mentalism tricks where they have to concentrate - adding things up, reading a series of little cards - was perhaps not the correct choice. These things are suited to spectators and situations where you really do have people's attention, i.e. they're not chatting to their friends in the mean time, and when they've not had much to drink.
Could you tell us more about what sort of performance it was?