Undersizing on running watts
The most common failure. The load list adds up, the unit arrives, and the compressor trips it on the first start because nobody accounted for the surge.
Ignoring site access
Gate width, overhead clearance, ground conditions and the fuel truck's route decide whether a towable unit can actually work where you need it. Check them before delivery day, not during.
Skipping the transfer switch
Backfeeding a building panel through an outlet endangers line workers and everyone in the building. A transfer switch or interlock installed by a licensed electrician is the only acceptable way to feed house circuits.
No fuel plan
A diesel unit running continuously will empty its tank on a predictable schedule. Deciding who refuels, when, and how it is billed after the unit arrives is how jobs stop at 2am.