Sometimes, major programming mistakes or poor attention to detail are found in a developer's work. In such cases, it should always be escalated to a full code review of all the work that developer has done. Worse, their work may have been used as a starting point for others.
Once you find some bad code, spend a little time to check if there is more:
“Liz was working on a large project with ten developers of various skill levels. She found some code where files would not be closed if there was an IO problem, because a finally block wasn't used. She then raised the issue with her team lead who asked her to explain the problem to the developer. She did a quick search for all other instances of similar blocks of code and fixed them too. The entire process took less than a few hours.”
