The Solidity assert() function is meant to assert invariants. Properly functioning code should never reach a failing assert statement. A reachable assertion can mean one of two things:
- A bug exists in the contract that allows it to enter an invalid state;
- The assert statement is used incorrectly, e.g. to validate inputs.
Consider whether the condition checked in the assert() is actually an invariant. If not, replace the assert() statement with a require() statement.
If the exception is indeed caused by unexpected behaviour of the code, fix the underlying bug(s) that allow the assertion to be violated.