Graphs: General-purpose graphs cannot be created as they may need to represent cycles. Cycles are impossible for the type system to reason about.

Bit twiddling: Overloading bits with multiple meanings. Examples include using the NaN bits in f64 for some other purpose or the higher-order bits of pointers on x86_64 platforms. This is somewhat common when writing language interpreters to keep representations within the word size the target platform.

Self-referential types are too hard for the borrow checker to verify.

Intrusive data structures: store structural metadata (like pointers to other elements) inside the elements themselves, which requires careful handling of aliasing.