I learned XXX as a placeholder for FIXME from a scientist who learned to write physics papers in the 1980s. I believe he learned it from someone trained in the early 70s. It flows well in text: "The measurement performed by XXX in 1985 observed the same effect, but at the XXX-confidence level."

I solve the XXX-as-stand-in-for-foo degeneracy by adding priorities to my XXXs: XXXRC, XXXMA, XXXMI, for release-critical, major, and minor bugs.

A simple grep | wc shell script allowed rudimentary but extremely flexible bug-tracking when I was writing up my thesis.