Let's say your application talks to a database. You reuse connections with a connection pool, but you accidentally reuse connections with different privileges

Let's say your application talks to a database. You reuse connections with a connection pool, but you accidentally reuse connections with different privileges and scopes. As a result, sometimes you get to read some data you shouldn't read and sometimes you don't. Or, concurrency bugs. You don't properly serialize transactions and sometimes two transactions overlap in time leading to conflicts.