It is a sunny day and a among his other wares a street vendor is selling umbrellas for $11.99. After you pass his stand it begins to rain, and you return to the vendor. The umbrellas are now priced at $15.99.

I want to say patio11 addressed this, but I may be misattributing. In any case the advice I recall is to, if anyone notices or complains, offer them a direct link to the lower price, possibly accompanies by a 10$ off coupon. An apology about the mix-up is optional, as is bringing up the fact that you are the founder, and you're happy to chat with customers any time on how to improve the service.

You should probably also charge all customers the lower of the two test price points at the final checkout step (as mentioned by whyleyc in this thread). That should avoid most complaints.