The math teacher in me is going to come out for a while, although I'll admit that it's really cool that it happened.
Those InstructureCon t-shirts are pretty nice. Most of the t-shirts I wear are branded with Canvas or Instructure although today I'm in one from USCOTS. If you're coming to InstructureCon, you may only pack Canvas t-shirts to save space.
How many photos did you take? The chance that you win the lottery is very small. The chance that someone wins the lottery is much higher.
If you had started off with the question "I'm going to take a picture of the Walt Disney statue in the foreground with the castle in the background, what's the probability that someone wearing a Cavnas t-shirt will be in my photo?" then the answer would be very, very small.
When you rephrase it as "I'm going to InstructureCon this year. InstructureCon is going to be near Disneyland and I live on the other side of the country (or world) and this may be my only chance to get to Disneyland, so I'm going to go. The conference has 3000+ attendees so it's likely that someone else from the conference will be feeling the same as me. The conferences have been going on for several years and they give away multiple high-quality t-shirts at each so there are a lot of t-shirts out there. Many people like to show that they've been to a conference before, so they wear previous t-shirts when they come back. People are proud to be associated with Canvas, so it's more likely to be worn. High-quality t-shirts are great to wear when outside in the summer. I know that I'll get a lot of swag at this year's conference and so I'm going to pack light so that have room in my luggage to bring it all back home. I love taking photos, so I will take a lot of them. What's the probability that at least one of the photos I take will have someone in a Canvas shirt in it?"
Now factor in that many things happen that we're unaware of, but when we see them happen, we assign a much higher significance to them. There's a very good chance that you saw someone in a Hard Rock cafe t-shirt, too, but it didn't register with you the way that an InstructureCon t-shirt would.
I really need to get back to the Thinking Fast and Slow book I'm reading that explains a lot of the psychology of the mind from a statistics perspective. Life happened this summer and I got distracted.
Setting aside the probability of it, I think the addition of the InstructureCon 2017 t-shirt makes it all the more special for you. That photo now has a significance that it may not had it just been a picture of Walt.