I've worked on this exact problem before. There are two kinds of possible errors: 1. On site errors- small errors that occur at the spot you intended to edit. O

I've worked on this exact problem before. There are two kinds of possible errors: 1. On site errors- small errors that occur at the spot you intended to edit. Occur maybe 30% of the time- usually will not break anything, but worth thinking about. What we know so far is that these are not random and it will be possible to rationally design crispr sequences to reduce (or even purposefully cause) specific on site errors. 2. Off target errors- this is when Cas9 cuts the genome at the wrong spot- this is the main error people are worried about. Computational efforts in guide design have attempted to reduce this, and will get more advanced as we learn more about the binding kinetics of Cas9. These also occur non-randomly, at highly quantifiable frequencies, and are due to biophysical rules. Also, newly discovered and engineered Cas variants have substantially better specificity and will work to further reduce this problem. Also off target effects are usually not problematic and can be tested for (will be bred out of crispr-ed crops)- however, if you're going to do CRISPR in live humans it is likely that regulatory bodies will require a demonstrated very low probability of off target effects. Basically, errors are an issue to spend a lot of time thinking about when designing a crispr system intended as a human therapeutic. For most scientists, however, errors are not terribly important or common and they don't need anything beyond a bit of rational guide design for their experiments.