The Harvest Forum would be the place to ask this question. The canning experts are there and they are very friendly, very helpful and incredibly knowledgeable.

I don't can green beans because we like the texture of frozen green beans better, so I hesitate to comment on a process I don't routinely use but I also don't want to give you half an answer or to just say "go ask on the harvest forum". (Really, though, the Harvest Forum is the best place to ask canning questions!)

In general terms, the world won't end if you forget to remove the air bubbles from a batch every now and then, so I wouldn't be overly worried about that. I'd just try to remember to do it in the future. The beans are not ruined as long as the seal is good when checked 24 hours after they were canned. When water does not completely cover the beans in the jar after they have been pressure-canned, as long as you followed all the pressure-canning rules and the jars did seal, the beans should be safe to eat. The food that sits above the water line may discolor though. If the jar was only half-full of water, then that would sort of be where you draw the line and either reprocess (if it has been less than 24 hours since the original processing) or discard the green beans uneaten.

If you check your jar seal roughly 12-24 hours after the food product was canned and the jars are not completely sealed, you can reprocess as long as you do it within 24 hours of the original canning process.

If ever in doubt about whether a food product is safe to eat, don't eat it. Discard it carefully, while wearing gloves and taking great care not to let the possibly tainted product make contact with your skin. Botulism is invisible and you will not see any signs of spoilage that would visually tip you off that something is wrong, nor would the food smell like it has spoiled.

Sometimes the few air bubbles is not the problem. It is usually the rapid change in temperature that pushes the water out. If your canner has the jiggler, and you turn the temperature down or up too quickly then that usually makes the water push past the seal. Use small temperature changes even if the jiggler is going bananas. It takes longer for the temperature inside the jar to change.

That being said. As Dawn said, if you followed the instructions, the beans will be safe to eat. Although they may be drier and discolored. Just add some water to them when you reheat to eat them.

I don't know if it happens with green beans, but sometimes with some starchy vegetables, the veggies can absorb so much moisture that your water level in the jar falls even if you didn't do anything to cause the water to vent out of the jars. There's lots of reasons it can happen, like the one Robert mentioned.

Let me find and link an article that discusses the various ways that water can "disappear" from a jar during the canning process. On the linked page from Pickyourown, page down about a half-page and you'll see the Q&A on water loss..

Sometimes people overreact to water loss and tighten the lids on subsequent batches so tightly that air cannot escape from the jars as the canning process is occurring, which is not what you want to happen. That's why you always want to remember to tighten the rings only fingertip tight but not incredibly tight....you do not want the rings to be tightened as tight as you can get them. I hope that makes sense.

Loss of Liquid During Canning