Strings
Defects in string comparison and construction, where the value semantics programmers expect of string diverge from how the type actually behaves. Strings are immutable reference types, so equality and concatenation each carry a subtlety that ordinary-looking code steps into.
Comparing with == against a value typed as object silently falls back to reference equality rather than content equality, and building a string by repeated + inside a loop allocates a fresh copy on every iteration — quadratic work that a StringBuilder collapses to linear.