The LineItem#stock_availability validation breaks admin when you try to increase/decrease the LineItem#quantity because the validation isn't account for units already purchased. So for example, reducing a purchased quantity from 5 to 4 when Variant#on_hand is 3 will result in a validation error, as well as increasing quantity on a purchased line item from 4 to 5 (buying one additional unit) under the same scenario.
Account for purchased units in stock validation
Looks like this got merged, I just didn't close this issue. Doing that right now :)