A really cool feature of mirrorless cameras is that the image you see in the electronic viewfinder is the same one that the sensor sees through the lens. (This is a step up from a DSLR, where you only see what the lens sees.) Moreover, camera settings that modify the captured image will likewise be shown in a mirrorless camera's EVF. This allows for fun things like making the X100s into a Monochrom, where you see an achromatic image while you're shooting.
This all works great, and even helps to diagnose problems like sensor dust: you can see crap on the sensor in the viewfinder, allowing you to take care of it by cleaning the sensor instead of cleaning up dust spots during development.
Seeing crap on your sensor when you're in a foreign country and don't have any sensor swabs, however, just makes nausea.
The X-Trans CMOS can sometimes be a crap magnet.
Such was my experience shooting in Istanbul. Switching lenses on the XE-1, I noticed something on the sensor that hadn't been apparent before. I pulled the lens off, gave a gentle blow, reattached the lens, and—still there. Rinse and repeat, and no better result.
I had cleaned the sensor before leaving the United States, but I honestly haven't had enough issues with sensor gunk to consider that I should pack some swabs for travel. My mistake. A panicked search of the web revealed what I already knew: there's no way to MacGyver cleaning a sensor. Sensors are too delicate, and need specialty cleaners.
This led to the next problem—finding a camera store that had sensor cleaners. Here's a pro travel tip: Google Maps doesn't translate, and it won't guess as to what you're looking for. So if you don't know how the locals write "camera store," and if (as is the case in Istanbul) the locals haven't added English terms to their business description, you'll get all kinds of very wrong results. (However, I did find the Lomography Boutique in Istanbul! (And no, they had no clue how to clean a digital sensor.))
Five stories of photography shops. Not bad!
Luckily, I found a very helpful discussion post on dpreview which directed me to Sirkeci, which is relatively close to Sultanahmet. A walk and a tram ride later, I found a mini mall consisting of five floors of camera stores. This was good. This was like B&H but spread out a lot more. But despite all the shops and the dozens of retailers of both new and used film and digital camera equipment, it took me four shops before I finally found one that sold sensor swabs. (Many had no qualms in pushing lens cleaning kits.) Find it I did, however, and it wiped my sensor problems clean.
Lesson learned: from now on, my travel kit will include a sensor swab. Yours should, too.
That said, if you find yourself in Istanbul and want a break from the crowds around Sultanahmet, take a walk across the street and start looking for the camera store signs. They won't be in English, but you'll recognize the goods.