Prints fail. Not occasionally, in the way a kettle occasionally fails — routinely, as part of the job. What changes with experience is not the failure rate so much as when you notice: most failures are decided in the twenty minutes of setup before the machine is even switched on.
It came off the plate
The most common one, and the most boring. A first layer that does not stick properly will let go halfway through, usually at the exact moment the print gets tall enough to have some leverage. Plate clean, plate level, first layer squashed a little more than feels right.
The supports were in the wrong place
Supports hold up everything that would otherwise be printed into thin air. Put them under a face and you spend an hour sanding away the scars. Leave them off an outstretched arm and the arm prints into nothing. Getting them right is judgement, not a setting, and it is most of the skill in preparing a resin print.
The model was thinner than the process
A wall can be perfectly fine in a 3D program and impossible in reality. If a spear shaft is thinner than the process can resolve, it either does not appear or appears and snaps during cleanup. This is the failure that most often turns into an email from us instead of a parcel.
If we look at your file and think it will not survive being printed, you hear about it before we start — not after. A quotation you have not accepted has cost you nothing.
It warped
Large flat FDM parts pull up at the corners as they cool, because plastic shrinks. The fixes are unglamorous: slower cooling, more contact with the plate, or splitting the part so nothing is both large and flat.
None of which is a reason to be discouraged. It is a reason to send us the file rather than fighting it at home — start here, and if the mesh is rejected there is a link on that page to send it to us as a custom request instead.