when they pressure their partners for sex before they're ready :/
For men its: getting fat.<br />
For women its: Letting their man get fat
For me, the biggest mistake would be infidelity. I always told my husband that the one thing that I could never forgive would be infidelity, even since before we were married, then we were just dating and what does he do, but go ahead and cheat on me and get caught, after 15 years exclusive together, 10 years of marriage and 7 years with kids, and it has now been three years and I still feel like it was just yesterday, and worry that i may not be able to ever get over it, even though i hide the fact that it still eats away at me every day.
Being dishonest.......if either partner decides that they want to fool around with other people then why do it secretly....be upfront and tell your partner before it happens
They don't communicate their needs or problems to their partner. Most problems in relationships come from lack of communication.
Unreasonable expectations, the inability to understand that no one is perfect
Falling into a rut and allowing the passion and love to be taken for granted.
Men wander, so men, if you aren't ready to commit DONT GO OUT WITH US IT WILL JUST HURT PEOPLE MORE THEN NOT GOING OUT!!!<br />
Women: Expect too much. Really we do, so girls we need to stop white picket fencing our lives:P
The first thing that comes to mind is pretending to be what they are not. I'm thinking mostly about that initial stage, when it's mostly about charm and romance. A person might pretend to be committed or interested in things they aren't. Or they might put on an act to seem as if they "have it all together". Whatever it takes to snag the relationship or the other person. Then later, when the truth comes out, there are all sorts of unpleasant "surprises". Like cheating or incompatibility. A sense of betrayal and that you never knew this person after all.
Vessa is exactly right, it is the initial lies or exaggerations that people tell to impress others.
getting bored. But it may be natural, more than a mistake.
Not talking about items in a relationship. Being honest about what each one expects prior to getting really deep in a relationship. Especially if it seems to be getting serious. There is a big difference in just dating and getting into a deeper relationship. That is where the long talks and being open need to come out. Before before it gets serious than after a few years of either dating or marriage.