After finishing my latest project, I've added this to the OP:
March Update: The Trouble with Microchips
We know and love them from LBP2, but don't place too much trust in the Karting Microchip - like inlaws at your birthday party, Karting Microchips will overstay their welcome.
I recently built a lot of objects with Microchips holding Sounds and Effects Emitters. I found that sometimes when I place one of these objects and then later delete it, the Circuit Board from the chip will reappear after I've exited and re-loaded the level. The chip isn't there, just the circuit board - and everything on it works, so in my case I had random sounds and effects popping up in my level.
The Circuit Board cannot be deleted. It can be closed, but that doesn't get rid of it permanently. All the logic has to be deleted individually. This isn't so bad compared to other problems in Karting, but if you've got a lot of logic on a lot of unwanted Circuit Boards, it's time consuming.
To make things easier, I recommend that any large Microchips be placed on another chip. This way if you have the glitch, you can just delete the large chip off of the other chip's Circuit Board. It's much quicker than stripping away loads of logic.
Edit - forgot to add this little gem:
And... When you've saved an object that includes a Microchip and then take that object from your popit, sometimes the wiring has disconnected itself. Most often this occurs when you have wires between two chips, but it can also happen within a single chip or between a chip and an object.
If an object you've saved has logic problems, check the wiring. I've created one object with some very simple logic and no matter how many times I redo the wiring and capture, the logic is disconnected next time I place the object.