You get a phone call from A, who you've worked with in the past, and they want to offer you a job. This is fantastic because A is an unfailingly brilliant colleague, and the job itself is something you're very good at. Not a life-changing career opportunity, but fun and enjoyable and worthwhile.
Browsing A's organisation's website, however, you spot that A's boss B is advertising a job which might be even more interesting, and certainly carries a more responsibility and challenge and onward career prospects: it will be very, very competitive though, and you'd be interviewing against people ten or fifteen years your junior flying in from across the world. If you apply to work with B though, A may have the rescind their job offer.
YABU: Stick with A's job. You know you'll enjoy it.
YANBU: Apply for B's job. It's a huge risk, but it would probably be worth it if you get it.