just wondering what would garlic do if left to grow for an extra year?
Every clove would sprout so you'd have a cluster of plants growing in a small area and both the plant and bulb size would diminish considerably. In the thread linked below I uploaded a photo last year of what it would look like if you leave garlic in the ground (it's the 8th post down).
Here is a link that might be useful: Why not plant garlic in Aug/Sept?
It so happens I did this (totally by accident) last year, and discovered two clumps of plants sprouting this spring from where I had failed to find and harvest the bulbs last year. Instead of letting them be, I separated each tiny plant early this spring and replanted with more space, to experiment. They are growing happily and should be ready for harvest in a month or two. If you are interested, I will be happy to let you know what my eventual results are regarding size of bulb.