I'm quite an amateur here, but I have always considered them just poorly aligned, with lowered value, unless the printed part is off the bill and onto the other edge. Poorly aligned bills are sometimes easier to find than perfectly centered examples. Some experts here can correct this in the likely event I am wrong.
In a nutshell, if the third printing (serial number, etc.) is in its proper position and the borders are of different widths (about the same on front and back) it is a cutting problem. If the third printing is shifted and the boarders are of different widths it is a printing problem. There are variations on this. If the back or front have different boarder shifts that is also a printing problem. Typically, if the top, bottom or either side of the next bill is showing on the note it is considered an error and worth collecting. Otherwise, it is typically a spender. There are also crooked printing errors.