Dust. It’s annoying to clean off the shelves you have, but it’s even more annoying to clean your own sensor. There are kits and tools to make it easier, although there isn’t anything easier than handing your camera over to someone who works for your camera manufacturer — better yet, it is free if you’re attending WPPI.
I’d check with your manufacturer to see if they’ve got a presence at WPPI first and then see if they’ve got their cleaning booth up.
I know for a fact that FujiFilm has their Check and Clean station that will gladly accept your bodies to clean dust off, update firmware and check functionality. They’ll also do a similar process for the lenses, providing firmware updates if available and checking functionality — deeper cleans than surface dust though will need to be sent out for service.
It took them 45 minutes to get my X-T2 and three lenses to get cleaned, but I took about an hour and a half roaming the expo before I remembered to pick it up. So one could potentially drop it off, attend a class, then pick it up later on!
Canon also has their CPS team out there to do free cleanings.
If you’re not at WPPI this time around, which ends on March 1, 2019, make sure to get your sensors cleaned next time around!