The project is in their computer

When code reviewing or pair-programming with juniors, I often point to a line of code or a method and ask: what happens when two instances on two different servers execute this line in the same time? Usually the answer is a starring face or a two servers?

Everything is fine

Usually juniors expect the application environment to be perfect, and even get surprised when asked about the behavior of their code when the database server fails, or the network disconnects, or a third party API returns an unexpected result..

User input is entered by the "user"

A junior sent me his work for code review and was very proud about the exhaustive validation he put on the user input. When I asked him why he didn't validated the User-Agent header, he argued that it is not entered by the user and it cannot contain special characters or HTML anyway. The best way to convince him was to inject a nice JavaScript alert in the admin panel through the User-Agent header.

All database queries are fast

When asking about the performance of a database query written by a junior, sometimes I get the answer: I tried it, it works instantly. Many young programmers don't see the difference between a development database server with few records and few developers connecting to it and a production database servers with millions or trillions of records handling hundreds of concurrent connections.