I was tasked with updating ReCaptcha v1 in an (6-7 year) old project that had MooTools 1.2.0 in it.
After RC was swapped, on a specific page it started exhibiting an error:
Once loaded, after a few seconds it would reload, then after a few seconds it would display a broken panel saying "make sure your browser is up to date" and "why is this happening to me". At no point was I able to check the checkbox.
After hours of trying to debug, I narrowed down to the MooTools lib, and the solution was to load MooTools after recaptcha was loaded.
I don't know if MooTools is still actively developed, or if this problem persists with more current versions, but maybe this info will help someone.