Here's one though that Halliday's [tense] system doesn't cope with:
if I had have done it
Ignoring the if makes it clear that this is simply an instance of someone misinterpreting
I'd have done it
I had have done it
I would have done it
The tense system provides the means of interpreting such instances — this one would be: "past-in-present-in-past" — and thus, the means of understanding why they are anomalous, and thus, why they are unlikely to scale the cline of instantiation and find a place in the general system of potential.