This came up in Question 3 from the first paper of 2005: "Outline the anatomical structures relevant to the insertion of a femoral venous catheter."
Here is a classic diagram to help one remember. This one is from Radiopedia, via Wikipedia (so, Creative Commons twice removed)
The femoral vein lies within the femoral triangle:
The insertion point of the femoral CVC is:
Ensure that the puncture of the vein occurs below the level of the inguinal ligament.
Puncture above the inguinal ligament is in fact a puncture of the external iliac vein.
The external iliac vein ends up being as deep retroperitoneal structure; if it is lacerated and begins to bleed, it will be impossible to place pressure on the puncture wound.
Embarrassing retroperitoneal haematoma will result.
If one selected the femoral vessels as a favourable site because of concerns regarding coagulopathy, a puncture of the external iliac vein will completely obliterate any possible haemostatic benefit. You may as well have gone for a subclavian vessel- at least a haemothorax is accessible via a chest drain.