It turned out that all of those disks were fine, so I continued downloading the other disks in the series by minicom'ing to my ISP's shell account, ftping them to the remote disk, and zmodem'ing them to my local disk. I played nethack on another virtual terminal while this was working.
Well, I switched back about 5 minutes later to check on the progress and it was just crawling, like 4KB every few seconds. I moved the mouse to hit the 'cancel' button on the zmodem transfer window, and the transfer rate shot up just then. I thought "well, okay..." and went back to nethack.
A few minutes later same thing happened. Move mouse, transfer speed goes up. I didn't understand IRQs at the time but I grasped apparent causality. I decided I'd try moving the modem to a different IRQ but that required a reboot. I wanted to finish the current disk set, so I sat there with a book in one hand, twirling the mouse in little circles with the other.
That's my hand-crank modem story.
To this day, whenever the network is slow, I twirl the mouse in little circles subconsciously.
 like these ( http://az413224.vo.msecnd.net/img/40934/m_40934_1.jpg ) not these ( http://images.yourdictionary.com/images/computer/_ACUPLER.GI... ).
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...