Often times the user approaches the app differently than intended and the app may give error, those cases are called edges cases and we have to solve those problems by looking and how many % of users it is affecting. typically 2-5% means it is something serious and should be addressed immediately. anything less than 1 can be ignored but if the app scale is substantially large it cannot be ignored for example in the case of Facebook 0.1 percent of users may seem small but in this case it is enough to be addressed immediately.

Thinking of an edge cases in interview etc is vital as it shows that you’re capable enough and know enough.