Based on the
jpegtran man page (and Wikipedia), the top left of a JPEG image must be at the corner of an 8x8 block, so it's possible to crop single pixel lines/columns from the bottom and the right edges, but not from the left or the top. Full 8x8 blocks can of course be cropped from the left and top too.
The man page mentions this under the
Like the rotate and flip transforms, lossless crop is restricted by the
current JPEG format; the upper left corner of the selected region must
fall on an iMCU boundary. If it doesn't, then it is silently moved up
and/or left to the nearest iMCU boundary (the lower right corner is