I've worked as a teller (and vault teller and new accounts rep) at a bank for nearly two years now. Some of the skills and knowledge I've gained are obvious -- I know what our different types of accounts are, for example. Here is a list of the ones I find most interesting.
1. Count money quickly. Okay, this one is pretty obvious, but I've gotten much, much faster and more accurate. When counting out to customers, I fan the money so they can see every bill. When counting to myself, I use the first technique in this video. Only faster:
2. Estimate money accurately. The other day, someone came in with a coffee can full of icky $1 bills. I asked how many they had, and they had no idea. I estimated 170. In fact, it was 171. (I'm much less accurate with coin.)
3. Recognize the distinct turning, head-bobbing movement of someone looking for something to tell them what the date is.
4. Tell at a glance how many are in a string of zeros, between one and eight. So if I have a number like 45000000025734, I can type it perfectly without stopping to count the zeroes. In general, I've gotten much better at tracking long strings of numbers and typing them without getting lost.
5. Identify unusual coins. My favorite is that I can pick out real silver half dollars, quarters, and dimes out of a giant pile of coin. The trick is to look at the rims--they're straight silver instead of a gradient. The color also has a different tone--but remember, I don't deal much with new coins. Most of those things are nasty, which can mess with the color.
6. Identify counterfeit. Money is made of 75% cotton and 25% linen, and has a special coating that allows it to react with counterfeit pens. This is why counterfeit pens aren't actually a great tool; that security thread, which is on the $5, $10, $20, $50, and $100. It's actually inside the fabric of the bill, which makes it difficult to fake and -- this is important -- it's in a different spot on each denomination, so you can catch it if a $5 is washed and $50 printed on top. It won't wash out if you put the bill in your jeans' pocket in the laundry. And all you need to view it is your eyes and a decent light source. Here's a quick reference guide to the security features of US currency. (But honestly, a lot of "counterfeit" is just movie money, which has the wrong texture, so you can tell by touch.)
7. Get people to thank me for giving them bad news. It's all in the phrasing! It's nothing bad you did . . . it's that you're doing them a favor by giving them a head's up for the bad thing that's not in your control (even if it partly is).
There's no doubt more, but those are the ones that come to mind as particularly amusing me at the moment. :)
