To be clear, the protocol can't change unknowingly to other observers of the chain. When I say side effects, this generally refers to options within the protocol. Censorship and enacting a form of "emergency power" entrenching the governance are some examples. Secret hard fork preparation is also possible, and post hardfork there is no trustless way to validate what the new protocol is.
Worse though, if you're not observing the chain (i.e. running your own node) then you can't tell in constant time in a trustless way if the protocol has even changed.