The terms are almost never used appropriately.

As has been mentioned, technically "inerrant" means without error and "infallible" means incapable of error. If I score a 100% on a test it is "inerrant" but does not prove me to be "infallible."

In common practice, it is just reversed as most evangelicals in mid-century held to inerrancy as the shibboleth for differentiating themselves from liberals. In the late 60s and early 70s some places such as Fuller Seminary dropped "inerrancy" in favor of "infallibility" which to them, anyway, meant less to defend.

Dan Fuller (one of the residents in my retirement home ministry), penned an article back in that time period making the case for "infallibility" ("Benjamin B. Warfield’s View Of Faith And History: A Critique in the Light of the New Testament --
The Evangelical Theological Society." (1968). Journal of the Evangelical Theological Society Volume 11 (11:75). The Fuller faculty's "change" was the topic of a chapter in Harold Lindsell's Battle for the Bible ("The Strange Case of Fuller Seminary"). He argued that surrendering inerrancy was the first step toward other theological concessions. In the final analysis, I believe that history has vindicated his argument, if not entirely his attitude at the time.