Duolingo says both Без лимону!, Без лимона! are correct. Is that right? Why?
It's a very hard topic called "Declination of masculine nouns in Genitive case with endings -а and -у" :D
I hate it, it's sometimes not intuitive.
I'd say "без лимона" when you think of it as an object with specific "borders" (like countable) and "без лимону" if I think of it as a material without a fixed shape (like uncountable)
But it might be that only one is correct. In that case I'd vote for "а" because it's a clearly outlined object...
Maybe we'd better just rely on dictionary for words we are unsure in and don't think much about it :)
another consideration is whether a lemon is considered animate or inanimate. If animate, then лимона, if inanimate, then лимону. Beyond this consideration, I like the comments by sagitta145 and Vinnfred.