Sometimes it's clear that one or both parties were willfully deceiving themselves and/or the other party. Sometimes it's that they were unconsciously deceiving themselves or the other party.
I know some folks who are getting divorced because 2 years into the relationship, one of them realized they were trans.
Someone else, it was 18 years in and after having (and raising) a kid together.
I used to work with someone that realized they were a lesbian after being married 25 years and raising a child together with their (male) spouse.
For the other party, it can be a bit difficult to not see that as some kind of deception. It certainly changes 'the deal' pretty dramatically.
Knowing a bit about what was going on, I was also seeing signs of emotional abuse in one or both parties that was concerning, and may have played a part. It's not normal for someone to have PTSD type reactions to basic office human interaction, for instance. They insisted things were fine though.
Other times it's patterns of infidelity that come out much later, or actions that start once the 'ink is dry' that are deeply manipulative and destructive. Like 'oh honey, that's great that we're married now. Oh, I forgot to tell you about that $70k in credit card debt I just ran up.'.
Other times, it's things like 'marrying for love', then leaving once the green card arrives, or have their degree paid off, etc.
Or they're building multiple lives without anyone knowing - until someone finds out.
Or they get deeply controlling and abusive once they feel they get enough leverage.
Or they start screaming at their partner and stabbing the counter with a butcher knife because the other partner asked them if they were ok one too many times....
Or any number of things.
People are... messy.
If for some reason you have a reason to spend time in family court (if so, I'm deeply sorry), watch the other cases. It can be very informative. The reality is VERY different from how it is usually portrayed. Especially if you have EQ and can read between the lines a bit.
My recent 'favorite' being the Mom who had a valid restraining order against her (due to prior incidents) who decided to violate it to the point she physically stepped on and hurt the fathers mom in front of their daughter and disrupted their daughters school event to the point she had to be physically escorted out by security. She got scolded by the judge for violating said order - and receiving no other penalty.
Despite admitting to it in open court, TO THE JUDGE, and actually adding details that made it clear it was even worse. After the father hesitatingly brought it up.
Eye opening is one way of putting it.