Whether through my recruiting endeavors or the private resume services I provide to job seekers, not a week goes by where I don't see the word "expert" somewhere on a resume or cover letter. "Expert in $LANGUAGE" isn't all that uncommon, and today the expression of expertise might be depicted through a horizontal bar graph where several languages or concepts are rated as "beginner", "intermediate", or (here it comes...) "expert".
When the source is a professional that appears to have deep experience in a subject matter, I'm not likely to even pause. However, it's exceedingly rare these days that the claim of expertise comes from someone fitting that description. I'd venture to say I see it from recent graduates and students much more often than experienced technologists, and that fact alone seems to demonstrate my point.
There are two issues with claiming expertise on a resume or job application.
The claim demonstrates that you don't understand what expertise even looks like. I'm sure many readers are at least somewhat familiar with the "10,000 Hour Rule" claiming that is the required practice time to master a subject. Whether one agrees with that theory or not is irrelevant, as most of those in the programming world would agree that it's highly unlikely for anyone to become an expert based on an undergraduate curriculum and a couple internships. A claim of expertise related to a robust programming language may even indicate that the source knows so little about the subject as to even be able to recognize what might qualify as expertise.
You are now a target for people who likely know more than you do. I can recall countless incidents when I sent a resume to a hiring manager that contained some expert claim, and a hiring manager responded with: "So this one is an expert, eh? Schedule him/her for a phone interview with $BESTDEVELOPER and let's find out!". Your claim of expertise has now raised the bar for your interview and evaluation process, and your candidacy will face a much higher level of scrutiny than those who are either more modest or more knowledgeable about their own limitations.
There are better ways to demonstrate expertise than a biased claim or a trendy bar graph. Developers who succeed in coding challenges or provide samples of past work for critique can leave little room for subjective interpretation by employers. Quantifying your experience along with some accomplishments is another way to show ability.
Think twice about claiming to be an expert. You're often doing more harm than good.