"I have to agree with another review that this is very fragile. No, it's not an issue with pieces being connected properly/improperly. The new ball joint pieces just don't connect as well as older sets and will often slip out of position. For example, we have the Spiderman and Iron Man mechs from previous years and they stay together much better. For whatever reason, the ball joints in this piece aren't as secure.
My almost 5 year old built this (with my help), and it was incredibly frustrating to have legs/feet/etc randomly fall off while attaching an arm or another part of the build. Again, no such issues with similar sets from past years."