If something comes back to bite you in the ass it means that something you have done or were involved with in the past creates a negative situation for you later on in time. This is an extremely informal expression that could be considered vulgar in the wrong company.

1. You shouldn't have made that promise to Debbie. If things don't pan out as you expect, it could come back and bite you in the ass.
2. If you don't do something to fix your medical issue now, it could come back to bite you in the ass further down the line.
3. Dropping out of college has really bitten me in the ass. I'm struggling to find a well-paid job.

1. Giving away such a large percentage of the company was a big mistake and now it's coming back to bite me in the ass.
2. We delayed in patenting the new design and now that decision is coming back to bite us in the ass.