So I am making final alterations to my most recent project, a small silnylon tarp. Before I toss my arms up in victory, I thought I would ask others if they have had aspects of their silnylon or other project eventually fail? I have read post (on other sites) and watched videos of DIYers showing off a finished project, but wondered how well the DIY gear held up in field testing? I am ok with some of my stiching looking, uh, awkward, but am wondering how strong it all really is. Each seam and each loop I sew on, I wonder if I should reinforce more, add more stichs, or will more stichs weaken it?

Anyone had or heard of experiences where some aspect of DIY gear did not hold up as well as hoped?

What I've done....
Look at how other things are made. Usually you find that you might be overdoing it on your own project. My two main concerns are that once completed, it will keep me dry (or warm), and it hold together. It will never be a professional job otherwise, and I am fine with that.
Presently I am working on a large (for me) project that so far I have screwed up twice... it happens. I've got too much into it to trash it, although I was very close to doing that once. When it's done it will be dry and will hold together, so I will get what I set out to accomplish. On a tarp or tent I seal silnylon with 100% silicone sealer that has been thinned down with mineral spirits. I get it to almost a liquid form and brush it on with a cheap soldering flux brush . They are cheap at most any hardware store, they are the right size and you can just throw it away when you're done.
Here's what's important to remember... when you seal the seams, it doesn't just seal them but it also locks the threads so they can't unravel. Silnylon is so slippery that the threads can start to unravel easily so this helps a lot.

Remember, you can always make alterations, corrections or additions as you see the need. Field testing might be in order.

At our house we often have to remind each other not to "over engineer" things. Or, not to solve problems that are not really problems.