When you have a chest/factory with default capacity, and you try to move a stack from your inventory to that chest/factory. It will give you a "inventory too small" warning and not do that even though there's enough space.
How to reproduce:
# put some low quantity stacks in your inventory
# open up a new chest with no linked cargo blocks
# shift click a stack, make sure its volume isn't higher than the default capacity of that chest
# stack won't move and it will give you a "inventory too small" message