When junior developers join my team, I find that they're often shy about critiquing anyone else's code because they assume they're the least skilled person, so they couldn't possibly find anything to be improved in anyone else's code.
I've considered sending developers like this a code review and including the note, "And just so you feel comfortable suggesting improvements, I've intentionally left in one mistake (not necessarily a functional bug)."
I've never done it because I worry it comes across as condescending. Like, "Obviously I'm perfect, but to give you something to do, I added a mistake, and now it's your job to go digging around for it."
Instead, I try to reassure junior developers that they should feel comfortable making suggestions and asking questions in the code review, even if they're not finding things that are officially "bugs."