Take, for instance, this question, which recently showed up on the ETJ list.
I was asked if the sentence "if he's not going to change his mind
there is no use talking to him" was correct.
I said yes.
"But you can't use if with the future tense" came the reply
"quite right" said I.
"so why is this ok?"
In case you're asking where these two came up with this "rule", here are a couple of versions for you. The first one is from an TESL site called englishpage.com.
Like all future forms, the Simple Future cannot be used in clauses beginning with time expressions such as: when, while, before, after, by the time, as soon as, if, unless, etc. Instead of Simple Future, Simple Present is used.
And here's another from Wikipedia (now changed):
Future tense forms are not used in the condition clause (protasis) in English: *If it will rain this afternoon, …
If you subscribe to the future-tense school of English grammar, it is completely natural to look for regularities in how such a tense would be realised. This is what we're seeing above. The people who are promulgating these "rules" are looking at a specific instance in which it seems to be true (e.g., *if it will rain this afternoon) and generalising from there.
But this disallows perfectly fine sentences such as "if you'll excuse me, I've got a bus to catch" or even "If you'll be using a wheelbarrow frequently, then make risers low". It also catches instances such as the one that initiated the question above.
On the other hand, it ignores other problems such as "it could rain tonight" becoming "if it could rain tonight...", or "that may be the right one" becoming "if that may be the right one", or even "he might have been there" as "if he might have been there".
A belief in the future tense does this because it leads people to look for regularities in some hodge podge notion of the future tense where there are none to be found rather than within the system of modal auxiliaries where they actually exist.
In fact, the problem only resides with certain modals (mainly 'will', 'may', 'might', and 'could') when they are used to express probability (i.e., epistemic uses). Thus, "it could rain tomorrow" doesn't work as *"if it could rain tomorrow", but "you could help me tomorrow, couldn't you" easily becomes "if you could help me tomorrow" because 'could' here is denoting ability (or willingness, i.e., it is deontic) rather than probability. Note that this also holds true for present and past time as well as future time.