These errors have been termed "ghost" notes.  They have either missed an application of ink or that application has been misplaced. 

Here are notes which have missed applications of ink.  Note as dramatic as the "ghost" notes, but still very interesting.

These also have missing applications of ink but somewhat create a "ghost" like appearance as well.  Us the thumbnails and see that a couple of these have very spectacular errors on the back as well.

The sheet which thess notes came from was shifted when the Steel engraving print (the last one) was applied. Click on the thumbnails and see that the printing is shifted in different directions.

During the printing process, a sheet can be missed being fed into the press and therefore the ink is transferred to the back plate causing offset printing.
Occasionally sheets have similar errors as these, but are caused by incomplete drying of the sheets or excess ink on the sheets as they are stacked together, therefore transferring the ink to the next sheet. Errors like these are ink transfers like above.

This note possibly could be a double denomination note. However, double denomination notes have one side as one denomination and the other being another. Since I have never heard of an actual double denomination Canadian note, this is the closest there is. As to how it happened, it is thought that perhaps the sheet of paper that this note was printed on, may have been used to set up a press and obviously before a run of $100.

During the printing process, areas of notes can have too much ink.  When the notes come into contact with each other, some of the ink can be transferred to the other note.

Here is a good one with a couple of errors.  At first one may think the serial numbers are misaligned, but have a better look by clicking the thumbnail.

A very rare error where the sheet was flipped between applications of ink and the serial numbers.

Here is an ink transfer error which is minor but actually fairly rare occurrence.

Termed an "wet trap error" this is a very interesting and rare error.

Here is an ink smear error which is minor and fairly common.

You will have to click on the thumbnail to see this error.  It is missing the orange print around the large number 10