Sometimes old ideas become worthwhile again because the hardware landscape changes.<p>An example from compilers: Register allocation for expressions (no cyclic dependencies) is an old and solved problem. For an expression like a+b+c+d+e+f, you wanted to order it like (a+(b+(c+(d+(e+f))))), because that minimizes register use. Then newer CPUs got out-of-order processing. Now you want ((a+b)+c)+(d+(e+f)), because in contrast to the previous order the CPU can now sum in parallel. Another case, where digging up an old algorithm helps.<p>What probably also happens a lot is that people reinvent stuff without knowing previous work.