High temperature and dryness, or high temperature and extremely high humidity are the things you have to watch out for.
If your instrument gets too dry, there is danger of cracking or splitting, especially on solid top instruments.
Hot and humid conditions can soften the glue in the instrument and can cause pieces to come apart, especially parts under tension, such as braces, the bridge or neck joint. (I had to practically rebuild a 12-string that was left in the sun in 90% humidity)
Humidity changes that are rapid enough to actually damage the instrument would be an unusual event. Going from 50% in the case to 80% in the house shouldn't cause anything more than having to re-tune the guitar as the wood swells a little.
Taking a guitar from next to a fire in a warm house and placing it outside in the snow would be bad for it; or the other way, taking a very cold and dry instrument and placing it next to a steam vent would likely damage it.
Opening your case and letting the instrument acclimate is fine.