Ontario construction businesses carry meaningful tax exposure on every project. The expensive mistakes are rarely exotic; they are recurring operational misses.
Common problems include claiming ITCs on holdbacks too early, misreading HST timing, vague invoice descriptions, missing registration thresholds, and co-mingling collected HST with operating cash.
Late remittances hurt because they are avoidable. Calendar discipline is boring, but it is dramatically cheaper than penalties and interest.
The broad fix is to make tax handling operational instead of reactive: cleaner invoice structure, separate tracking for holdbacks, and a habit of reserving collected HST before spending.
This is general information, not CPA advice. But for contractors, simply building a cleaner compliance workflow removes a surprising amount of friction.