That is actually incorrect according to page 34 of the MCP2515 data sheet, RXFnSIDL bit 3 only allows one, or the other. Two filters need set to receive extended and standard frames if RXBnCTRL (page 27) bits 6 and 5 are set to ‘00.’ If those bits are set to ‘11’ then the MCP2515 ignores the masks and filters altogether. The section I commented out in the library to get the chip to receive everything was setting RXBnCTRL bits 6 and 5 to ‘00.’ Since figuring out the masks and filters, I’ve restored that section of my library.