I ran some code that generated a large list "data" (2.6GB). At some point I aborted the evaluation. Subsequent executions would seemingly freeze the system. Due to a lack of physical memory virtual memory usage was loading the system. I could see the kernel "had" over 2.6GB even with nothing running. Clearly that memory was from my aborted run. So I typed Clear[data]. That had no effect. Was my original tlist no longer global? Was it forgotten by the front end?
The quick way out was to exit and restart Mathematica.

Lesson learned: Make big data local to a Module unless it must be global. Use Clear[] for global data. But aborting evaluation can leave orphaned data.