GST filing has a reputation for being mechanical, pull the numbers, submit the F5, done. Most of the real errors happen specifically because it’s treated that way, since several common mistakes are about what shouldn’t be claimed, not what wasn’t entered correctly.
Claiming input tax on disallowed expenses. Not every GST paid on a business expense can be claimed back. Medical expenses for staff (unless they’re contractually obligated under certain conditions), club membership subscriptions, and expenses related to private motor vehicles are specifically disallowed categories, regardless of how clearly business-related they look on the invoice. A business that claims input tax across the board without checking the disallowed list is one of the more common sources of an IRAS query, and it’s rarely deliberate, it’s usually just that the disallowed-category list isn’t something finance teams check often enough to remember by heart.
Missing a valid tax invoice. Input tax can only be claimed with a proper tax invoice showing the supplier’s GST registration number, not just any receipt or informal invoice. A real expense, correctly incurred, still can’t be claimed without the right paperwork behind it, and this gap often isn’t noticed until a GST audit asks to see the underlying invoices, by which point getting a corrected one from the original supplier can be slow or, for an old transaction, no longer possible.
Getting deemed supplies wrong. Gifts to customers or staff above a certain value, and goods taken for personal use from business stock, can count as a “deemed supply” requiring GST to be accounted for even though no money changed hands. This is one of the least intuitive parts of GST, and it’s the kind of thing that gets missed entirely rather than miscalculated, nobody thinks to apply output tax to something that was given away for free.
Partial exemption apportionment errors. A business making both taxable and exempt supplies (certain financial services, for instance) can’t claim input tax in full, it has to be apportioned between the two, under a formula that isn’t always applied consistently once in place. Getting the apportionment wrong either overclaims input tax or leaves legitimate recoverable tax unclaimed, and both outcomes tend to persist quietly across several filing periods before anyone checks.
Missing the reverse charge on imported services. GST-registered businesses that aren’t entitled to full input tax recovery may need to self-account for GST on certain services bought from overseas suppliers, under the reverse charge mechanism. This applies more narrowly than people expect, but when it does apply, it’s one of the more commonly overlooked GST obligations, precisely because the supplier never charges GST on the invoice, there’s no line item prompting anyone to think about it.
What actually prevents these. Checking input tax claims against the disallowed-expense list as a routine step, not an exception; confirming every input tax claim has a matching valid tax invoice before it’s included; and reviewing deemed supplies and reverse charge exposure at least once a year, since these are the categories most likely to be forgotten rather than miscalculated.
If any of this applies to a filing already submitted, voluntary disclosure to IRAS before an audit catches it is generally treated more favorably than being found through a review, worth raising if something here sounds familiar.