I created an autoload script named MCP. I later said to myself, "Self. That was stupid. It should be a parent class." So, I deleted MCP from the autoload list, saved the contents of the .gd file, and deleted the .gd file. I then tried to create a Node2D named MCP with a script of mcp.gd, and the script had a "class_name MCP" in it. The editor did not like that in any way, shape or form. Wound up naming it Mcp.
Nothing named MCP exists in my project, but if I enter mc, then the editor continues to show MCP as an autocompletion. It's like it never left.
Does the editor have a cache of some sort I can clear?