My friends Bob and Nancy have been together since they were old enough to crawl and share pacifiers. After three kids and 35+ years of marriage, they're still stuck on each other. Such longevity is enviable-and rare. Most of us have an ex or two lurking in the shadows. And while I prefer that the past stay right where it is, that's not always possible, especially when there are blended families. When everyone gets along, it's a beautiful thing. Unfortunately, that too is a rare thing.

The ex factor is exponentially complicated. To demonstrate:

Mark and Sharon arrive at a wedding to discover that Sharon's ex-husband Joe and Mark's ex-wife Carol are there with their respective spouses. Through some colossal blunder, all six of them are seated at the same table. Let's take a look at some common mistakes partners make in these situations:

The Memory Lane Blunder. Mark and Carol split on good terms and, as old friends are inclined to do, begin a stroll down memory lane-whatever happened to . . . remember that trip we took to the Grand Canyon . . . did you ever get that hernia repaired . . . and so on. Both Sharon and Carol's new husband are left out of the conversation. Not good.

The It's Okay With Me Blunder. When Sharon attempts to redirect the conversation so she and Carol's husband can participate, Carol responds with a terse comment, then asks Mark about an old friend. Mark cheerfully answers, continuing down memory lane and the path to Sharon's how-dare-you-let-her-treat-me-that-way wrath. Really not good.

The New Best Friend Blunder. Sharon's relationship with Joe ended bitterly. Sharon ignores Joe but engages in conversation with his wife who acts as though Sharon is her new best friend, going so far as to suggest they get together for coffee. Joe is fuming at what he perceives as his wife's disloyalty.

The Inside Joke Blunder. Joe, angry at being ignored, interrupts Carol to ask Mark if Sharon still sleeps in beat up old T-shirts and has learned to cook. Mark replies honestly about the T-shirts and makes a joke about Sharon's cooking. Everyone, except Sharon, laughs. Mark is headed for death row.

The It Wasn't My Idea Blunder. Sharon, feeling ignored and hurt by Mark, is furious with Joe and shouts, "Why are you being such a bastard? It's not as though I left you. You're the one who wanted a divorce!" Table talk becomes an awkward silence.

Remember:

When dealing with your sweetheart's ex, take your cue from your sweetheart. If your sweetheart's relationship ended cordially, be cordial. If it ended bitterly, be polite but keep your distance.

When dealing with your ex, take your cue from how your ex treats your sweetheart. If your ex is friendly to your sweetheart, be friendly. If your ex is rude, be polite but keep your distance.

It pays to remember that your sweetheart is your today and, hopefully, your tomorrow. Keep your foot, your thoughts, and above all your attention out of the past and on your sweetheart.