Walls in Revit are designed to automatically join when they get within a close distance. This is usually a good thing except when you don't want them to join, then it can be frustrating. Here how to override this:
1. Drag the wall end back from any other walls.
(please see comments for a better step 1. Thank you commentators!)
2. Click on 'Edit Wall Joins' tool.
3. Click on the end that you don't want to join.
4. Click on 'Disallow Joins' in the options bar.
5. Move the wall back into position (it won't recognize the other walls)
The 'Edit Cut Profile' tool is useful when used in combination with this technique. (We'll talk about that tool another time.)