My inability to remember which is the correct spelling – whether or wether – means I accidentally write about castrated rams rather more than anyone outside Farmers Weekly should ever do so.
@SimonWoods "Bellwether" is a related word that we're more likely to know. But many of us then misspell it as "Bellweather".
@JohnBrady Ah! I knew it sounded familiar. Without knowing I would have first taken a stab with one "l".