L1

No program decides termination for all programs. The ancestor of every other undecidability result.

Some infinities are bigger; there are more problems than programs. The proof technique that The Halting problem and Godel's incompleteness theorems both reuse.

The serial fraction caps your speedup, brutally. Pure algebra, not an observed regularity — the empirical part of the speedup story belongs to Gustafson's law and...

Every observable behavior of your API is depended upon by someone; and the robustness principle that made that worse, plus the modern case against it.

Any algorithm that shrinks some inputs must grow others. Kills every "compresses random data" claim on sight.

n+1 items, n boxes, one collision. The cheapest impossibility argument in existence, and the seed for No universal lossless compressor and The birthday bound.