Here are some of the pitfalls we experienced (in no particular order):
1) We took pride in our ability to figure out the game and come up with the basic layout of the robot really fast - like Saturday afternoon - so we would have plenty of time to prototype and build the ill-conceived fruit of our haste. Faster isn't always better.
2) We built parts and subassemblies (gearboxes, etc.) when we could have used kit parts or bought suitable COTS parts. Its tough to admit that our home-grown ideas aren't always the best way to go.
3) We built the "low cost team field elements" and didn't consider how they differed from the "real" ones. For example, in 2010 we only looked at the steel crossbar on the tower when thinking about our lift concept. The wooden 4x4 support posts didn't inspire any thoughts about a curling lifter.
4) During brainstorming, we restricted our technical approach to concepts that were "possible". "Possible" was defined as something we already knew how to do. This is where the rookies really help. Reality hasn't beaten the creativity out of them yet
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5) We tried to be too sophisticated, and built machines with too many potential weak links. They were pretty useless unless everything was working perfectly - which was rare. KISS!
6) Our robots were hard to drive and operate. They made unrealistic demands on operator skills and cooperation in a match situation. KISS again.
Hopefully, we learned something from our past mistakes. However, its still hard to tell the difference between "great", "good enough", and "fail" during build season. We can study rules, develop a strategy, list desirable capabilities of the robot, and build prototypes. Like most teams we go through this process in a mild vacuum and we won't know if the machine we decided to build was the right one for the game until we get to the competition.
In general, the best robots do the essential tasks quickly, reliably, under difficult circumstances, and are easy to operate. KISS (done properly) is one common thread that ties these all together. Ingenuity is another, but we haven't figured out how to impose that one on ourselves yet.