If More Filler is Needed at a Followup Appointment to Correct Uneveness Who Covers the Expense?

It seems that many offices have the patient pay for additional filler product if the results the first time were not done evenly. With as much as fillers cost, it seems reasonable that people would expect the fillers to be placed evenly the first time, and if they were not that the office would pick up the expense to correct the uneveness since the patient trusted the injector to place it evenly the first time. I'm not talking about if more filler is needed on both sides to add more volume, simply when things turn out uneven. Who do you think should be covering the expense of the product in this case, and why?