Groepaz from the VICE emulator team and the C64 demo group Hitmen just released an updated PDF of 6502 illegal or perhaps more accurately called unintentional opcodes.

I would say this document and the use of illegals are for advanced programmers but they can come in very handy for generating smaller or faster code at times.

Some illegal opcodes can be unstable on certain chips

NMOS 6510 Unintended Opcodes no more secrets (v0.91 – 24/12/16)