There's been a bug identified in the system that prevented very large contraptions from saving (Or more accurately, it would save corrupted and would not load afterwards). It seems to occur at around 140 objects in the level, including level objects and contraption objects.
As a stopgap measure and temporary solution Pacifist has updated the game so that it does not allow you to place the 121st object into your contraptions. It will not display an error message, the objects will simply not draw.
Pacifist is on vacation so it might be a while before he gets a "real fix" in removing this limit.
If you run into this building cap, feel free to save your contraption, you will be able to continue working on it once we get this resolved!
Big heaping apologies to anyone who has lost your contraptions to date.
(remember to clear your browser cache to get the new 120-limit version)