Given that this is a site for us Nikon fanboys, and that Canon seems, these days, to be the brand that most prominently rivals Nikon; this thread would not be complete without someone calling out the brand of your printer, and claiming that that's what is wrong with it. Perhaps even mocking you for admitting to owning a Canon product.
We're a collective of mature adults around here, Bob. That sort of thing simply doesn't happen.
Oh, wait...
...
Ha ha! Yes, it's a CANON printer, and a pretty decent one at that! I think Canon was rather smart to branch out into printers. If DSLR's wind up not being as popular down the road, at least Canon has their printer sales going for them.
So...whatever was going on last night has resolved itself.
Last night I turned off the printer. This morning after turning it back on, it went through the head cleaning process even though it did it last night. Then I removed the cartridge and carefully looked around inside plus inspected the cartridge once more. Nothing unusual showed up so after reinserting the cartridge, the printer went through yet another head cleaning process.
I did a test photo using 4x6 paper. As a precaution, I left at least a 1" margin all the way around, and it came out without any splotches around the border area. So I tried out an 11x8.5 letter size photo (that's the size where the problem showed up last night), and it came out perfectly. Then I did a second letter size photo--same excellent print again!
Obviously the head cleaning must have cleared up whatever was wrong. Many thanks for the information!
I will keep watch for this in the future, and at least now I have an idea what might be causing it should it occur again.