Materials and suppliers
Trade accounts, hardware runs, fuel and hire charges across multiple suppliers, most of them on statement rather than invoice.
You are on site all day. The bookkeeping is whatever is left at nine o'clock, and it usually is not much. That is how a trade business ends up three quarters behind without anything actually going wrong.
The volume is not the problem on its own. It is the volume combined with nobody having time to touch it until a deadline forces the issue.
Trade accounts, hardware runs, fuel and hire charges across multiple suppliers, most of them on statement rather than invoice.
Invoices arriving in every format imaginable, with the employee-versus-contractor question sitting behind them.
Money invoiced but not yet received, and retentions held long enough that they drop out of everyone's memory.
Vehicle and plant purchases, chattel mortgages and hire purchase arrangements that need to be recorded properly, not as an expense.
Materials paid for now, the job invoiced later, and the customer paying later still. The gap has to be visible before it bites.
The ute, the phone, the odd personal purchase on the business card. Easy to do, tedious to unpick a year later.