Shortest Error Message

ed, 3 bytes

Note: Most of the answers here are ignoring the trailing newline printed as part of the error message in their count. But I don’t see anything in the question to justify ignoring it, and the author commented that the newline should be included. So unless the question is changed, I’m going to include it.

Code (with trailing newline):

??

Error (with trailing newline):

?

C (modern Linux), 19 bytes

Would've done my famous segfault but totallyhuman stole it.

main(){longjmp(0);}

Output (18 bytes):

Segmentation fault

Python 2, 35 bytes

import sys;sys.tracebacklimit=000;a

Gives error:

NameError: name 'a' is not defined