I've photographed plenty of surprise proposals, and finding a way to be near the action sneakily without being noticed has become easier and easier.
Normally, what will happen, is the client and myself will talk over the details of the location where they will ask the question, what they and I look like so there is no confusion, and where I will be hiding or out of sight for the big moment. For this particular shoot, we goofed a few things unintentionally.
On this day there were a ton of photographers on the canal. There were engagements shoots, wedding party shoots, and graduation shoots all over. We had set the location of the moment being at the back exit of a museum that was on the canal. However, neither of us realized there were 2 back exit locations. While I awaited the couple to exit from one side of the museum, while trying to be inconspicuous, they were coming out from the exit a little further down the canal walk. My client mistook another photographer for me, and popped the question thinking he was getting coverage! A big "Oh no!" moment!
When he realized the mistake, the two walked down to try to find me. I see them approaching, and try to not make eye contact, and act like I'm taking photos of something else. I was quite surprised when my client called out to me, and thought, "No, the surprise will be ruined!" Then, he explained to me what had happened.
I felt so, so bad! Miscommunication abound!
However, we decided to make the best of the situation. We used the hour we had together to get great engagement photos. We even staged a proposal picture, just for fun.
This situation shows a big truth, mistakes happen. No matter how many years you've been doing something, or how much you know or are prepared, an accident or a mistake can and probably will arise. How you react to it, and what you do to fix or make it work is what's important.