These connectors are cracking very easily:
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Actually I'd call that a Tehnic pin, as does BrickLink. bricklink.com/v2/catalog/catalogitem.page?P=32556#T=C – chicks Apr 17 '17 at 3:05
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This happens if the parts are under a lot of strain, usually as a result of unevenly distributed weights or improper gear assemblies.
Also make sure that the parts are being handled gently and teeth aren't being used to remove them. Since these can sometimes be hard to remove, students (especially younger ones) often try to use their teeth to grip them while pulling them out. This will almost always lead to the pegs breaking down.
