Thanks for all your imput regarding this note!
After reading your comments, I'm quite convinced that this is an actual printing error. The ink does cover a very large portion of the note, from the left of the security strip to just above the W in W.P.Jenkins. Although it is possible someone actually spilled ink all over the note while trying to stamp it, that hypothesis just doesn't agree well with the pattern of ink on the note.
Also, when I examine the color of each pixel in photoshop, the colors of the smear resemble very closely the colors in mac's suit. For both, the darkest areas show up as black rgb(0,0,0) followed by very dark blues (0,0,30) or so... and the lighter areas continue to have a blue dominance eg. (100,100,125). I really can't tell the two apart.
And what fun would it be spending $10 to keep it if I wasn't an actual error??
The serial number is BTA1505998 and the back is perfectly normal.
Any ideas on how commonly ink smears from excess ink would occur?