Why do some people lie, or cover up information, when being a shidduch reference? What happens when information is not disclosed and the couple gets married?

What needs to be disclosed (e.g., having a colorful background, health problems, addictions)?
When and by whom?
How should we respond to shidduch inquiries when asked?
Should references disclose everything? Can they lie?
Are people typically lax in disclosing?
Do certain segments of Judaism fail to disclose more than others?
Can things be patched up when problems are discovered after marriage?