1912, 5c Deep Blue & Black,
With Cordoba Portrait of 2c (40a). Faint cancel at top leaves entire
design clearly visible, deep rich colorsVERY FINE AND CHOICE. ONE OF THE MOST UNUSUAL ERRORS IN ALL OF PHILATELY. ONLY NINE COPIES ARE RECORDED.
Little is known of how this error was made or why there are so few of them. The discovery of this error is described in an article by Edward S. Conger in the 1952 Philatelic Congress Book. The first copy was discovered in Paris in 1925, where it was supposedly found in a packet mixture of Canal Zone stamps. It was brought to the United States by a dealer, who sold it to Eugene Costales of Scott Stamp & Coin Co. It was first written up the July 1925 issue of Scott Stamp Monthly, which started a frenzy as dealers and collectors checked their inventories for more examples. Other cop[ies were found in New Zealand, Reading Pa., Toronto, and in the Canal Zone.
Ex Plass. Illustrated in Canal Zone Stamps on p. 92. With 1966 and 1993 P.F. certificates. (Image)
1915, 1c Green & Black
(46). Mint N.H., variety "Z" of "Zone" under "C" of "Canal", Very Fine
and choice, Scott Retail as hinged (Image)
1915, 1c Green & Black, Double
Ovpt. (46b). Tropicalized original gum, overprints well apart, wide
margins, Very Fine and choice, scarce, only 180 printed, two panes were
printed, one with overprints close together, the pane with overprints apart
also produced two other varieties: Scott 46c "Zone" double (two) and 46d
double overprint one reading "Zone Canal" (18)
(Image) 
1915, 5c Deep Blue & Black
(48). First printing with "E" of "Zone" under "L" of "Canal", barely
hinged, deep rich colors, Very Fine and choice, this spacing variety
occurred on only three positions of the first printing, Scott Retail as
normal (Image)
1917, 12c-24c Views of Culebra
Cut (49-51). Top right corner margin blocks of twelve, most stamps Mint
N.H., left vertical row of all three some perf separations sensibly
reinforced, rich colors, few light natural gum bends, Fine-Very Fine,
attractive set of matched blocks, Scott Retail as blocks of four (Image) 
1918, 1c Green & Black, Ovpt.
Reading Down (52a). Sheet margin at top, part tropicalized original
gum, choice centering, Extremely Fine (Image) 
1918, 1c Green & Black,
Booklet Pane of Six (52b). Slightly tropicalized original gum, wide
margins, rich color, fresh and Very Fine, accompanied by stapled booklet
cover (Image)