Hi!
Occasionally I have encountered this issue and solved it by methods Jonathon Ziegler suggested in another post in this thread.
But almost always I found the errors to be my mistake or snap not working entirely correctly.
Let’s start with human error. You don’t say, but I will guess your triangles are two different fill colors (if they were the same, you would have used Pathfinder > Add/Unite, right?). The first two things I look at are did I accidentally leave a stroke on either path? If I did I remove it.
Next is snap: I have guides, smart guides, and snap all turned on (usually). Sometimes snap just gets you “close”, not exactly on the mark. Zoom in as much as your version of AI lets you and see if the anchors actually are right on top of each other. If not, move them.
Frankly, the second is more likely than the first these days… I’ve messed up so many times I check my settings before I draw. 😉
Now, if neither of these are the case, then I do as Jonathan suggests: I compensate for Adobe’s limitations. Which brings to mind one last possibility: If one or both of your objects intentionally have a stroke it is a bit tougher, but solved similar to the snap problem by zooming in and adjusting your anchor points until the results look right.
I hope this helps!
Steve Crook, Jr.
http://www.stevecrookjr.me
I am a simple creative professional that can get my Adobe suite and a few other creative tools to do what I want. Barely. 🙂