Some good answers here:
Gotchas that can get the unready:
A contour interval of 100 feet or 20 meters is common on mountain maps. Even a slope that shows an even gradient can have lots of stairsteps too big to handle on close inspection.
It's easy to see cliffs from the bottom. Much harder to spot from above. Plan your down route while going up. If you are doing a loop, put the hard part going up, and come down the easy route.
Watch for low elevation traps. The last thing you want is to be perched on top of a cliff with no ready way around at the end of the day, tired, and facing the night. This makes for bad decisions.
If you really want to find a way up, prepare to come back several times, and do a series of scouting missions.
Don't rely on being able to come down on the other side. One of the more frequent mountain formation process is the block thrust. This view looks north. Coming up the west side is a cakewalk. The east side is a bit tricky.
Give yourself lots of time.
Have a plan for loss of visibility, and/or lightning.
Every few minutes turn around and look back the way you came.
Have two means of navigation -- e.g paper map AND GPS.
Leave your plans with someone who will expect a call or text when you are back in contact with the world, and will raise a stink if you don't call in.