The regulations state that pilot must follow all ATC clearances and instructions,
and request clarification anytime the pilot is unsure of the clearance or
instruction to be followed.
This should not be taken to mean if ATC tells you to do something, that you have
to do it. The controller has may have no idea if you are capable of
complying with a clearance. They just know they need you to do something.
You are the pilot and they trust you to tell them immediately if you cannot
safely comply with any of their instructions by using the magic word "UNABLE".
There is nothing wrong with telling a controller unable anytime you do not feel
completely certain you can safely comply with the instructions to be followed.
Regulation also states you may deviate from your air traffic control clearance if
an emergency exists, to the extent necessary to handle the emergency. If
an air traffic control clearance is deviated from, the pilot must notify the
controller as soon as possible.
If a pilot in an emergency requests priority handling by air traffic control, the
pilot must submit a written report of the emergency within 48 hours, if a
report is requested by air traffic control.
Finally, one important aspect of this regulation forbids a pilot from operating
according to a clearance issued another aircraft. It is important to
listen carefully on the frequency, since it is easy to mistake a similar
sounding call sign for your own. If you take another aircraft's clearance
by mistake, you have violated this regulation. If you're not completely
sure of what you heard, there is no harm in asking the controller for
§ 91.123 Compliance with ATC clearances and instructions.