Nearly everyone has something in their career background that they really don't want to appear on a resume. It may be something innocuous like a mediocre college GPA or it may be something significant such as a prison term. A resume should be written to highlight the value of a candidate and downplay bits and pieces that would play against the candidate. That is why strategy is such a huge part of designing and writing a resume. Most people don't think about strategy when they are writing their own resume; they are simply thinking Did I get down what I do? Strategy is key, though, and using strategy to your advantage is what a winning resume is all about.
I have seen resumes where people have taken pains to try to hide things in their past that they don't want prospective employers to be aware of in the selection process. Some issues are truly red flag issues while some are more mountains out of molehills and are only significant in the minds of the job seeker. Some true red flag issues include the following:
Prison Term - I know you keep thinking I'm harping on this issue but I've seen many professionals who messed up at one time or another in their life and now don't want to trumpet their mistakes to the world. One client in particular stands out in my mind. He was a pharmacist who ended up with a significant prescription drug habit following a bad car accident. He was downing the inventory and the authorities got wind of it so he thought burning down his pharmacy was a wise decision. He spent five years in the can for it. He was out of prison, clean and sober, when he came to us and wanted to start a new career in IT. Dealing with that five years and a career change on a resume was a challenge.
Job Hopping - Frequent job changes are considered a red flag issue to employers. It could mean many things, not all of which are negative. The negative causes could be the candidate is incompetent in his job, is difficult to get along with, or just jumps at any offer of a slightly higher salary. Employers sink a great deal of money and time into new hires and they would like to get their money back in productivity before the employee moves on so they steer clear. A possible positive cause of job hopping is that the candidate is a rising star - someone who is extremely talented, well-known in the industry and highly recruited.
Time Gaps - Time gaps are best handled by telling the truth. If you've taken time off to raise children or care for elderly parents, say so! By trying to hide it, you make it into a red flag issue.
Education - The same holds true for education, especially lack thereof. Many people will leave off the education section altogether if they don't have a college degree even if they have 25 years of experience with all kinds of training. Leaving off the education automatically draws attention to it and creates a mountain.
Notice I haven't listed age as a red flag issue. That's because like lack of a college degree, it's a molehill rather than a mountain. Sure, age discrimination exists as does nepotism and gender bias and every other kind of bias you can think of. It's not rampant, though. In fact, with the Boomer generation retiring, there is a wisdom drain occurring in the job market and those who have been there, got the t-shirt are looked at as good investments.