I recommend to my clients that they carefully consider whether they would want to know about a potential disclosure item if they were purchasing the property. If the answer is "yes" then it's a good candidate for disclosure.
Some situations are a "no-brainer" -- for example recent mold discovery and remediation should always be disclosed. Known asbestos in the house? Disclose! Inoperative appliances, faulty electrical, plumbing issues, roof leaks? Disclose! Pets in the house? Disclose! A death on the property within the past 3 years? Disclose! Death of someone from AIDS in the house? Technically, one doesn't have to disclose this situation, but a good idea to disclose anyway (and if asked specifically by the buyer, definitely disclose!). A new freeway on-ramp being built next to the house within the next few years? Disclose! Known sexual offender in the neighborhood (on Megan's List)? Disclose! Added a room, installed a new water heater, furnace, roof, pool, plumbing, or electrical without proper building permits? Disclose! Loud noises in the neighborhood (neighbors, trains, planes, motorcycles, freeway noise)? Disclose! Poor cell service in the house -- I would disclose.
But there are also gray areas -- the roof leaked 5 years ago, was professionally repaired by a licensed roofer and hasn't leaked since? Likely not critical to disclose, but wouldn't hurt -- providing documentation that it was repaired goes a long way as well -- and shows that you have nothing to hide. A roof leak that you repaired recently, but that may or may not have been thoroughly rain tested? Definitely disclose! Cracks in the slab under the flooring that you don't know about or have never seen? How would you know to disclose? But if you saw the cracks in the floor when the flooring was being replaced, then disclose!
When a buyer closes and takes possession of a house, it is not uncommon for them to find things (mostly small) that were not disclosed by the seller and/or did not become known during a home inspection. In many cases, the seller was not aware of the situation, and therefore did not have an obligation to disclose. However, if the buyer learns, from discussions with the neighbors for example, that a seller knew about a particular item, and did not disclose it, the buyer may have recourse against the seller. So always better to over disclose than under disclose!