I like the confession of mistakes idea. But in the interest of brevity, I only mention my successes. But seriously folks... My mistakes are numerous enough to organize into categories:
1) Over-elaboration (OE) - this might be called over-design - or complexification. In every design project, I notice that I start with something too complex but nowadays I try to hone it down into something simple. At one time, I left things over-designed with predictably disasterous results. Examples: attempting to make alcohol stoves with closable reservoirs (a la Trangia) or remote fuel supplies(as in “call the fire department!!!”); Solution, Pop can stoves. Bivy tents - bivy sacks that erect like small tents to create a little headroom. Internal framed and external framed packs (various ingenious designs including carbon fiber framesheets); solution, go lighter and eliminate frames entirely. Multiple use sleeping bag with zippers to turn it into vest to replace jacket around camp. This had a huge fiddle factor and never worked to my satisfciton. Solution, serape-type neck hole in center of quilt (from Jack Tier of Jacks'R'Better).
2) Combining incompatable materials. Imagine a light day pack made of 4 ounce nylon with seat belt webbing straps and reinforcements. The bag hung like wet toilet paper from the almost rigid straps. Any combination of a heavy, stiff fabric with a lightweight, flexible one. That kind of thing. But usually more subtle. The worst mistakes of my 'mature' period were putting excessively heavy zippers on things - until I learned that #3 coils will handle almost anything.
3) Over-reinforcement. Strengthening items beyond the general capacities of their materials or design use. This one is a little subtle, but generally, reinforcements are useful only when failure at that point will preceed and/or cause failure elsewhere in the structure due to the limits of the dominant fabric. If you made a tarp of tissue paper, no amount of corner reinforcement would save it from disaster. Over-reinforcement makes stuff too heavy and less packable, obviously, but it can also weaken a structure by directing stresses to the wrong areas or by concentrating stresser that would be spread out by a weaker reinforcement.
4) Over-building in general. With modern materials it is entirely practical to make gear that will last several generations. For the last 50 years most backpacking gear has been retired not because it is worn out, but because its design becomes obsolete. Losses are due primarily to accident, abuse, misuse or getting lost in the garage. The exception is gear coated with polyurethane – which deteriorates sooner or later. Manufacturers overbuild because it is the easy way to avoid returns. Heavier fabrics also have a better ‘hand’ or feel. Rarely does performance dictate their use. So, yeah, I have a lot of excessively heavy gear. It gets used for utility duties such as storage bags and covering the lawn mower.