I did not know about the per day favour ceiling. I just donated a fair chunk o' sparkly to Cosma, got the warning, but thought it was "the usual" warning about no more XP for donations. Then spent ages working out if I'd accidentally given it to anyone passing...
Sigh.
It's not a bug, and the text warned me. I just...wanted to share my grief. Or something approximately two or three rungs down from grief.