The client who has fired two agencies in two years. Sometimes the previous agencies were bad. More often the client cannot be satisfied by advertising, and you are third in a sequence that continues.
Conversion tracking that cannot be verified before signing. If nobody can tell you what a conversion is today, there is no version of month three where performance is agreed rather than argued.
The person paying is not the person evaluating. Common in franchise and multi-location work. Without alignment, you are optimizing toward two contradictory definitions of success.
Budget below the floor where the channel can work. A $600 monthly Google Ads budget in a $40 CPC category is not a small campaign, it is a guaranteed failure with your name on it.
And the client who wants to talk to the fulfillment team directly, immediately. That is not a white-label engagement, it is a referral with extra steps.