Definitly the importance of letting other people test the game before the deadline. I was very much surprised when a lot of people said the controls were hard in our game. I was of course playing the game to test for bugs, so I naturally got good at the mechanics and therefore misjudged the experience someone would have when first touching the game - which is kind of an important part, especially in a jam ^^"
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Yup, this is pretty common in my experience. I work with physics a lot so it becomes easy to avoid mistakes when playtesting. Of course, if someone does end up screwing up then it needs to be forgiving enough to get them back on track. Perhaps that's the real issue instead of the game being "too hard to control" but I'll have to take a spin myself to see.