I tried to turn off as many things as I could earlier, but was still having the problem. Now, after suspending the laptop at work and resuming it at home, I can't reproduce it any more! I'm starting to think that this has something to do with stale network drives, even though I don't have auto-save turned on. At work, I can't reach my home network drives and those were mapped. I will see if I can reproduce the problem tomorrow and then fix it by deleting my drive mappings.
For reference, though, here is Capture Settings, Capture (thank you TextSnap!)
Delay time before the capture: 500 milliseconds
Auto-scroll refresh time: 20 milliseconds
Default background color for captures: Transparent
X Include cursor image
X Turn off font smoothing for capture
Play sound when making the snapshot
X Hide this window before capturing screen
X Restore this window to front after a capture
Enter full screen presentation mode after a capture
Do automatic paste, print or save ONLY if a new capture
is different from the previous one.
X Clear transparency for Window Captures under Vista
Mark new captures as "modified" (to prompt for saving)
I also have "Replace image in the current window with the new capture" set and no auto-save.