MicroWorlds can run a maximum of 20 processes at a time. Processes include: 

•
launch and forever instructions typed from the Command Center or inside procedures 
•
buttons
•
turtles and colors programmed to run instructions
•
selected text launched with Run from the Gadgets menu.

The Cancel item in the Edit menu only shows the processes that have been launched by forever and launch instructions. Use Cancel to selectively stop processes that aren't needed, or use the Stopall menu item to stop all processes and launch only those that are connected to what you are doing. 

It is possible to start processes by accident. Check that you don’t have a button running a procedure containing a forever instruction. Use forever with short instructions to be repeated continuously, and use launch with longer instructions.