What are the effects of living 12 years with unfinished root canal?

Potential disaster!: These types of situations almost always lead to losing the tooth. Worse case scenario is infection in jaw that spreads to other parts of your body resulting in serious complications and has resulted in death. Get the treatment completed if still possible or the tooth extracted. Get all your other dental treatment done as well!

It's a time bomb: You are lucky! you need to get this tooth evaluated to see if the tooth is still viable or are there underlying problems that haven't manifested themselves yet. You could have a chronic infection, you could have a tooth that needs to be extracted. This is a ticking time bomb that needs to be addressed.