What's the most confusing part of GST/HST for Canadian small businesses?

We recently launched a free GST/HST calculator for Canadian contractors and small businesses, and building it meant digging into the actual CRA rules around GST and HST - which surfaced a lot of small details that trip people up.

The one that surprised us most is the "place of supply" rule - the GST/HST rate follows where the work is actually being done, not where the business is registered. So a contractor based in one province but working a job in another needs to charge that job's provincial GST or HST rate, not their home province's rate. Easy to get wrong if you're not paying close attention.

Curious what others get stuck on with Canadian sales tax in general. Is it the GST vs. HST distinction? Knowing when provincial sales tax - PST, RST, or QST - stacks on top of GST/HST in provinces like British Columbia, Manitoba, Saskatchewan, or Quebec? The $30,000 small-supplier GST/HST registration threshold? Something else entirely?

Would love to hear what's actually confusing about GST/HST calculation in practice, versus what we'd assume from just reading the CRA site.