Noted as part of #3907, the inputs on pivot for quiver is very permissive and will take 'mid' or 'middle', however it will also work with out exception with any string input (by just silently falling back to 'tail'):

X, Y = np.meshgrid(np.linspace(-np.pi, np.pi, 16), np.linspace(-np.pi, np.pi, 16))
v = np.sin(X)
u = np.cos(Y)
plt.quiver(X, Y, u, v)
plt.quiver(X, Y, u, v, color='b', pivot='ardvaark', alpha=.5)

I propose that a) we validate the input to be in {'tip', 'tail', 'mid', 'middle'} and accept but warn anything else down to tail for one version before raising.