1925, 15c Gray (90). Left
plate no. 14912 block of six, bottom left stamp "Canal" wrong font
variety, barely tropicalized original gum, rich color, Very Fine and
choice, small backstamp, Scott Retail as normal
(Image)
1925, 15c Gray, "Zone Zone"
Ovpt. (90a). Position 18 in block of 28 with plate no. at right and
natural s.e. at top, barely tropicalized original gum, rich
colorFRESH AND FINE. ONLY THREE EXAMPLES OF THE 15-CENT "ZONE ZONE" OVERPRINT ARE RECORDED. A GREAT RARITY.
According to Canal Zone Stamps, official records indicate that all 250 panes from the lower right portion of the sheet were destroyed. However, three are known, including one in a full pane (ex Tows), which has not been seen for some time. It is possible that these were part of a shipment sent on a later date with second printing stamps.
Ex Plass. Illustrated in Canal Zone Stamps on p. 154 (detail). With 1967 P.F. certificate. (Image)
1926, 17c Black, "Zone" Only
(91a). Mint N.H. with sheet margin at bottom, almost perfectly
centered, bright shade, Extremely Fine Gem, only 20 were printed, this was
caused by the overprint being shifted to top on two panes and only occurred
on the bottom row of the sheets (Image) 
1926, 17c Black, "Canal" Only
(91b). Plate no. 18021 at top, stamp Mint N.H., natural gum skip at
top, intense impressionFINE. THE UNIQUE PLATE NUMBER SINGLE OF THE "CANAL" ONLY OVERPRINT ON THE 17-CENT SCOTT 91. A WONDERFUL POSITION PIECE.
According to Canal Zone Stamps, this error occurred on one pane when the overprint was shifted too far downwards. It also occurred on one stamp due to a foldover, and is occasionally found on some shifted overprints from the bottom row of a sheet or sheets. There are perhaps a total of fifteen in existence, including at least one with a trace of the word "Zone". The example offered here, with plate number at top, is a great rarity. (Image)
1926, 20c Carmine Rose
(92). Top plate no. 16648 block of six, barely hinged, brilliant color,
fresh and Extremely Fine (Image)
1926, 20c Carmine Rose,
"Canal" Inverted (92a). Position 48 from the upper right pane in block
of 25 with plate no. 17530 at right, two stamps at top lightly hinged,
error Mint N.H., few sensibly reinforced perf separations in
selvage, deep rich colorFINE AND RARE. ONLY TWO EXAMPLES OF THIS OVERPRINT ERROR ARE RECORDED.
According to Canal Zone Stamps, both examples are in blocks with plate numbers. The other example is from Plate No. 17531, and both have similar centering.
Ex Plass. Illustrated in Canal Zone Stamps (detail). With 1967 P.F. certificate (Image)
1926, 20c Carmine Rose, "Zone"
Inverted (92b). Position 76 from the lower left pane, in block of 25
with plate no. 16648 at bottom, stamps Mint N.H., few sensibly reinforced
perf separations at bottom, one stamp tiny thin spot not mentioned on
accompanying certificateFINE AND RARE. ONLY THREE EXAMPLES OF THE "ZONE" INVERTED OVERPRINT VARIETY ARE KNOWN. A GREAT RARITY.
All three known examples of the 20c "Zone" Inverted error are in large multiples such as the example offered here, and all have similar centering. Interestingly, they come from three different plates.
Ex Plass. Illustrated in Canal Zone Stamps on p. 159 (detail). With 1967 P.F. certificate (Image)
1926, 20c Carmine Rose, "Zone
Canal" Ovpt. (92c). Position 91 in block of fifteen with bottom left
corner margin incl. siderographer's initials and plate no. 16648 at left,
few sensibly reinforced perf separations, intense colorFINE AND RARE. ONLY THREE EXAMPLES OF THE "ZONE CANAL" OVERPRINT VARIETY ARE RECORDED.
Unlike many other overprint varieties of this type, this one was caused not by a shift in the overprint, but rather by the mistaken use of the slugs on the overprint form.
Ex Plass. Illustrated in Canal Zone Stamps on p. 159 (detail). With 1967 P.F. certificate. (Image)
1926, $1.00 Violet Brown
(95). Lightly hinged, Position 51 with "Canal" wrong font, wide
margins, Very Fine and choice, scarce variety, Scott Retail as normal (Image)