People would buy cheaper double density disks, drill a hole in it, so that the same diskette could be used as a 1440KB disk. Of course, they were of a far lower quality, and 'arj' (which was popular at the time) would often fail after the n-th disk.
Edit: heh, there is even a reference to drilling holes on the Apple website ;): http://support.apple.com/kb/TA39910
I would take one disk flipped over the other, mark the notch with a permanent marker, then cut out the outline. Most disks, like Elephant Memory, would work fine. You just flipped the disk over and inserted back into the 1541 to read the reverse side.
It helped when you were putting out hundreds of disks of pirated games on a regular basis.
I still think the single-sided disks were from the same production line as the double sided ones, and it was all market segmentation.
They called them flippy disks...
I'm guessing that 3.5 disks were binned, so maybe you could get away with it once in awhile? Of course, the only way you could find out was to actually lose data...