1. All addons disabled except TTT.
2. Be dismounted.
3. Open spellbook
4. Hover over any spell, such that the "CTRL+C to copy spellID" text becomes visible
5. Close spellbook
6. Any amount of time passes. 1 second or 1 hour, doesn't matter.
7. Mount on a dragonriding mount
8. Observe that all keybinds for dragonriding abilities are tainted and no longer work.

The opposite scenario also breaks - Start mounted in step 2, dismount in step 7, observe that keybinds for regular class abilities on the main action bar are tainted.

And disabling the copy on ctrl-c module (and reloading) fixes it? 🤔

oh that's silly of me 🙈
thanks for the tip, will fix it tomorrow, and see if I can do something to prevent it in the future ^^

Yes, confirmed disabling that modules fixes it.

Simply changing the hooks in OnSpellbookUpdate to SecureHookScript instead of HookScript also fixes it.