Should one say "Checkouted" or "Checked out"?
At the time of writing (2020), most people would agree that the latter is preferred, and that the former is either "wrong" or at best "dubious".
This will eventually change, and perhaps already has by the time you read this.
Indeed I can be fairly confident about that, much as it pains my own sense of what's "right", because there's an inevitability about regularization.
Looking back 50 years to 1970, nobody then would have even thought of saying "checkouted", much less said it out loud, for the simple reason that it was a two-word phrase, a verb and a preposition: "check out".
This is what happens when we take two words and mash them together to make a new word.
To begin with the new word is only used as one part of speech, most commonly as a noun. The other parts of speech remain as a hyphenated term or separate words.
Some similar examples of incomplete language evolution include:
- break in → break-in → breakin, but is breaking in & has broken in;
- break out → break-out → breakout, but is breaking out & has broken out;
- check in → check-in → checkin, but is checking in & has checked in;
- check out → check-out → checkout, but is checking out & has checked out;
- log in → log-in → login, but is logging in & has logged in;
- log out → log-out → logout, but is logging out & has logged out;
- set up → set-up → setup; but is setting up & has set up (or more recently, has setup).
- take off → take-off → takeoff; but is taking off & has taken off.
Such changes are much more rapid when a phrase falls into dis-use and then returns to popularity; the new generation learning it as adults have no parental reinforcement to fall back on, so use the most natural conjugation.
It takes a long time - or sometimes never - for the single new word to gain regular conjugations of its own. Sometimes different meanings for the same word will take different evolutionary paths; a common idiom will usually keep its existing form, while a new idiom using the same word may use more regular forms.
Regularization is an unstoppable linguistic juganaut; every generation complains how badly the following ones use language, and how weirdly the previous ones used language.
It only remains to be seen whether a literary form such as has takeoffen will supersede has takeoffed. In the unlikely event that this comes to pass, you read it here first...
Coming from this comment, somebody should put this gist on a calendar entry for 2050 and come back here to write what the officially used term is by then