When you roll the dice at the end of a roll of filament and start the print…and pray … close enough, the print completed with a few cm to go … \o/
I know that feeling well!! I’d look at the estimated length of filament and measure the remaining filament I had coming out of the extruder. In one case I was so stingy on plastic that I paused the print at the absolute last second, switched to relative coordinate mode (G91), moved Z up 10mm (G1 Z10), moved by some x/y offset (G1 X50 Y50), pulled back the idler, pulled the plastic out manually, put another also-small piece (a yard or so) in manually until it came out of the extruder, let go of the idler, G1 X-50 Y-50 to move it back, G1 Z-10 to move it back down, G90 to resume absolute mode, and then clicked Resume. 
@Jeff_Keegan
OMG. You’ve done that too?
Yup. I had a spool of PLA that I unfortunately tried unwinding from an existing spool onto one of those yellow extension cord spools… This of course left the outside diameter of filament on the inside of the new spool, and the inside diameter on the outside. After a month or two I looked inside the spool and saw maybe like 40-50 clean breaks in the plastic filament… As the plastic unwound from the spool, more small sections of plastic fell. Some of them went around the spool a few times before breaking, so I had a bunch of roughly meter or two long pieces of plastic that I didn’t want to waste. It was awful. I’ll look for pics.
Found a pic… Here was a view inside the side of the spool… Not good. http://www.keegan.org/jeff/homemovies/miscimages/misc-2013/IMG_8120.JPG
Ouch! Toast.