Most people realize that when they get involved with another person, it's highly likely that person will have a "past". And usually that past means other relationships — which means an ex lurking somewhere on the planet.

But what if that old, expired relationship refuses to relinquish its hold on your new love? What if the ex has her or his claws so deeply embedded in your love interest, that the past keeps seeping into the present? Sometimes, it's an unwilling bond; your love is fighting it every step of the way. But more likely, the situation is ambiguous. Both people are on some level unwilling to let go, whether it's for hate, love, obsession or revenge.

Sometimes, exes are so potent, they can't be fought, and before you know it you've switched places with the ex. Suddenly, you're the one who's out the door.

Originally, Carla met Paul in college. He was intense, forceful and focused. He was also completely head over heels over Carla. But she was still recovering from a break-up, and wasn't ready for Paul.

Fast-forward seven years, and Carla finds herself at loose ends romance-wise. She decides to call Paul on a whim. The results are instantaneous. A whirl-wind romance, which involves meeting his parents, looking at houses and discussing the future. It turns out Paul has never forgotten Carla, never gotten over her.

She is flattered, excited. Casually, during one of their many long, intimate conversations, he reveals he was involved in an on-again, off-again relationship with Jill for nearly four years.

"But it burned out," Paul confides. "It was so over, there was nothing left." Cold ashes, he says, before kissing Carla.

Carla agrees to think seriously of engagement on a starry summer night. The next morning rolls around and she can barely wait to call Paul the next morning. She's decided. She wants a future with him. A funny thing happens, though.

Paul can't be reached, at home or his office. Filled with a chilly foreboding, Paula drives to his house but comes up empty. She's too embarrassed to drive to his office, so she keeps calling...and calling. Finally, she does something desperate: she sends him a telegraph: "What's going on?"

By now she's more furious than hurt. Her dreamy feelings for Paul have all but evaporated in the sizzling heat of her outrage. Paul finally calls. Unapologetically, he explains that Jill came by to see him the very day after he and Carla were talking engagement. He and Jill "realized" they were meant for each other and decided to take the plunge. They're getting married in three months. Carla is not invited, by the way.

The moral of the story is this: If it ain't over, it ain't history. The trick is figuring out if that ex is truly old news. It might be wise to run a few system-checks before concluding your honey's old relationship is really over. They might get mad if you keep asking, but you may just find out the truth.