I have always thought that chain dropping was caused my not have FD properly set but now I am thinking this is not always correct. My FD is adjusted fine and shifts great but I notice I have dropped the chain 2 times in the last few weeks. No problems as I do not shift under load for FD and all I did was not panic and shift the FD and the chain went back on fine no problems and no chain suck.

So I assume this can happen even when adjustments are all correct and what about chain catchers? I am thinking I might just get one for a piece of mind.

Many will say that if things are truly adjusted properly and you shift properly, you should not drop your chain. I have a chain catcher (Amazon.com : Third Eye Bicycle Chainwatcher with Universal Clamp : Bike Chain Deflectors : Sports & Outdoors) and have not dropped the chain once since installing it, I think it is well worth the money and grams.

i have personally never dropped a chain on a properly adjusted fd. chain catchers do offer a nice peace of mind.

Chain dropping is a byproduct of the way index levers work. The lever uses an escapement which allows the spring to quickly snap the cage to the low fear position. That's fine when the chain is coming from the outer rear sprockets and has an outward bias. But when the chain is coming from the inside sprockets, it has a tendency to move inward, and the quick action of the FD cage can add to that causing overshooting.

When shifting manually, riders with a bit of experience develop a bit of touch (without even knowing they do) and ease the chain over more gently which prevents chain drop.

Shimano is very aware of this, and incorporated that touch into Di2 to so it shifts aggressively when the chain is coming from the outside, and gently when it's coming from the inside.

OTOH- while chain drop is more common, it's still not common enough to be an issue for most. If it is, or if you're concerned about collateral damage to a carbon frame, you might want to invest into some kind of chain keeper. For my part, I try to do the front shift earlier in the shift sequence and that seems to do the trick.