Photoshop :: Select All / Copy Or Flatten - Row Of Pixels Missing?

In some CS6 PSD documents (but not all), when I Select All (no feather) and then Copy, the result in the clipboard is one pixel smaller in height than the original—the bottom row is missing. This is verified by pasting, selecting the layer's contents, and reading the Info window, or by starting a new document which defaults to the clipboard image's size. One pixel too small. In addition, when I paste on top of the original (which usually my intention) you can see everything shift up by one pixel. (Showing that the image has been cropped at the bottom, rather than scaled.)
 
I am viewing everything at 100%.
 
Any guideline as to when this bug does or does not happen? I can't find a pattern--maybe it's any Copy that can fail and not just Select All? It seems like it may happen on a previously-scaled document (which I know messes up the text tool semi-permanently). But I hate having to go through the extra step of double-checking the dimensions of every single copy and paste. I'm really trying to get somewhere near my old CS3 productivity, but it's a struggle with all the new bugs!
 
(This is different from the bug where rectanbgular selections loose one pixel of vertical when you release the mouse button at 100% zoom, although it does sound related. That longstanding bug happens 100% of the time, but at least you can see in the Info box that it has happened—one more constant act of double-checking that PS6 work seems to need. Whereas in this bug, the Info window shows the correct full size of the document after you Select All—but when you Copy, the result has lost data.)
 
I see now that in the same document, something similar also happens when I simply flatten the document: a single-pixel row turns white... but at the top.
 
Flattened/background layers seem to be immune from the copy/paste height loss, so I thought I'd just flatten the image I want to copy... but the flatten itself loses data.