Here's a small tip that may explain the 'mystery dropped rib stitch' that you discover when you remove the ribber.
When transferring stitches to or from the ribber needles, you have to stop after you do x number of transfers and advance your yarn carrier before you can continue.
I find it can make a big difference exactly where you stop the yarn carrier with each move.
The red arrow above shows, where I stopped the yarn carrier re the position of the latch on the last rib needle that prepared to knit. If I happen to bump the handle of my crank while fussing about with the subsequent stitch transfers is it very easy for the yarn to slip out of that rib needle by the red arrow, and the latch drop to boot. Then when the yarn carrier is advanced, that stitch will drop.
Much safer - is to always stop the yarn carrier so that the last rib stitch engaged has the latch fully closed and the yarn well under the following cylinder needle.
Between my general klutziness and outdated tri-focals, bumping the crank handle is fairly common for me. By paying routine attention to where I stop the yarn carrier with the ribber on I save myself a lot of grief.