If I use my Axfell Militia Boots on my character (Monk 8/ Wizard 1), which casts Expeditious Retreat (5) Unlimited Uses/Day, I gain a normal speed increase as expected. However, if I use them again within the duration of the original spell, the effect seems to stack, furthering his speed, resulting in the movement speed of a level 20 monk. Rather hilarious, but unintentional I presume.