I definitely remember that one usually says: si quis veniret … and not: si aliquis veniret. But the recent question about quo quisque est sollertior and similar forms brought the following rule from Allen & Greenough to my attention:
The indefinite quis is rare except in the combinations sī quis (if any), nisi quis (if any . . . not), nē quis (lest any, in order that none), num quis (ecquis whether any) and in relative clauses.
Si, nisi, ne, num are certainly uncontroversial, but I seem to remember that there was a longer list, so I looked it up in my pocket grammar (Langenscheidts Kurzgrammatik Latein, if you must know), and indeed it lists the following words calling for the ali-less pronoun: si, nisi, ne, num, quo, quanto, cum. And google-ing further, I also found other lists, such as: si, nisi, ne, num, quo, ubi, quando, cum.
And indeed I find, for example, this from Cicero (Pro Rabirio postumo 36):
Ubi semel quis peieraverit, ei credi postea, etiam si per pluris deos iuret, non oportet.
Once somebody has sworn falsely, one must not believe him any more, even if he should swear by multiple gods.
It is generally understood that in all cases where you can say quis (or quid), you can generally also say aliquis (or aliquid) for emphasis. But where is it permissible (and needed, unless one wants to express emphasis) to leave out the ali- prefix?