Twice in 12 hours, I learned the same lesson.
My sister-in-law Christina is in town for BYU softball camp, so last night we decided to hit the Spanish 8 theater to see Kung Fu Panda. The main reason that cheap little Melanie was willing to go is because it's only $4.50 on Monday nights, so I thought we should capitalize (instead of paying regular price, $5.75). We each brought exactly $5 cash, and were surprised when the cashier said it would be $11.50 for both of us. My jaw dropped. I asked her if Monday wasn't $4.50 night, but coolly replied that the offer didn't apply to Kung Fu Panda. After searching the car for change, we raced home, grabbed my wallet and were back at the theater. Only now a line of teenagers had formed. Another register opened and we jumped over there. This cashier said, "That'll be $9." Apparently she didn't get the memo about Kung Fu Panda not getting the special deal. Laughing, we passed over our 9 dollars, not believing that we went all the way home for nothing.
Then, this morning, I went to the ATM to deposit some checks. While pulling up to the ATM, I realized I did not have a pen, since I had only grabbed my debit card instead of my entire wallet. So I wasted a trip to the bank.
So I think I finally learned to take my wallet with me everywhere. Just in case.