Can a test be added for this? Just checking h_axes and v_axes aren't negative should work I think.

Actually this is inconsistent with the behavior when the figure is too small even in absence of title/labels, which is to raise through SubplotParams.update:

fig, axs = plt.subplots(2, 2, figsize=(.5, .5)); fig.tight_layout()

raises ValueError: bottom cannot be >= top.

The inverted axes behavior only occurs when a title/label is present and the figure otherwise large enough:

fig, axs = plt.subplots(2, 2, figsize=(1, 1)); ax = axs[0, 0]; ax.set(title="foo"); fig.tight_layout()

so it should raise an exception too (... preferably, it should route through the same mechanism). Or the PR that fixes this should also make SubplotParams.update not raise an exception, for consistency.