What do the No True Scotsman and Slippery Slope fallacies have in common as reasoning errors?

Both protect a claim by an illegitimate move — one re-defines terms to dodge counterexamples, the other assumes a chain of dire consequences without justifying it.

• No True Scotsman — when a counterexample threatens your generalisation, you redefine the category to exclude it. "No Scotsman puts sugar on porridge." "My uncle does." "Well, no true Scotsman does." The definition is silently moved to keep the claim unfalsifiable.
• Slippery Slope — you reject a first step by claiming it must inevitably lead through a chain to some terrible end, without showing the chain's links actually hold. "If we allow X, then Y, then certainly catastrophic Z." The error is asserting the cascade rather than demonstrating it.

The shared defect is evading proper scrutiny: No True Scotsman immunises a claim against any counterexample by tinkering with the definition; Slippery Slope wins an argument by asserting an unproven causal chain. Both swap an actual justification for a rhetorical move.

Tip: A slippery slope can be a legitimate argument if you actually establish each link. It's only a fallacy when the cascade is merely assumed.