Hi All my TH'ing Friends
As with everything that is printed there is an acceptable margin and an unacceptable margin when it comes to US Currency. As this bill is a older bill I would keep it even with the stain. The Bureau of Engraving and Printing has specifications for acceptable margins but as for describing this as an error, not quite. Had the margin not been present or cut into the actual bill it would be considered an error. The thing you would need to check is to see if someone 'trimmed' the note by using a paper cutter to shear off some of the bill to give it the appearance of an actual error. currently the sheets are printed in 32 note groups, then the sheet is halved. You can purchase sheets directly from the Bureau of Engraving and Printing, or you can purchase partial sheets, i.e. half sheets (16 notes) etc. You can purchase on-line to save you the expense of traveling to Washington, DC or Fort Worth, TX. Both facilities have gift shops and they sell wonderful stuff. They don't sell US Stamps any more as they no longer print them, but they have teamed up with the US Mint to sell proof and mint sets, as well as the other US Mint offerings. Keep looking for errors as they are out there. One thing to remember is every error you find is one that slipped by the Quality Assurance Inspectors.
Thanks
Dave Mork
THing4CSA
PS:Remember; Don't leave it in the ground!