blundering, erring, flubbing, fluffing, fouling up, fumbling, goofing (up), lousing up, messing (up), screwing up, slipping up, stumbling, tripping

noddingbobbling, botching, bungling, butchering, foozling, mangling, mishandling, muffing, murderingmiscalculating, misconceiving, miscounting, miscuing, misdeeming, misgauging(also misgaging), misjudging, mistakingmisconstruing, misinterpreting, misunderstanding

dropping the ball, laying an egg