If you have a lot of units under your control and selected -- be it spiderlings, treants, whatever -- your units all show up in the UI on the bottom left of the screen as small boxes. (duh)
If you have 10+ spiderlings, this quickly becomes a large column.

Here is the issue: Even when units under your control are NOT SELECTED, you can right-click or left-click the part of your screen where the units WOULD show up (if they were selected) and you will select that unit.

Moving around with brood mother and randomly selecting ONE spiderling over and over caused me to find this glitch.

Original thread can be found here: http://dev.dota2.com/showthread.php?t=10821