What's the problem?
If I generate a BitArray from a bytes object i get a strange bahaviour of 
prepend:

>>> arr = BitArray(bytes = b'}i\x01\x1d0\x00\x00\x06\x80\x00', length=9, 
offset=27)
>>> arr
BitArray('0b111010011')
>>> arr.prepend(bits)
>>> arr.bin
'11111111111111111111111100000011'

What is the expected output? What do you see instead?

If i do the same thing coming from binary there is no problem:
>>> arr = BitArray('0b111010011')
>>> bits = BitArray('0b1')
>>> bits *= 23
>>> arr.prepend(bits)
>>> arr.bin
'11111111111111111111111111010011'

Which version of bitstring are you using (bitstring.__version__), which
version of Python (e.g. 2.7, 3.2) and what platform
(Linux, Windows, etc.)

bitstring: 3.1.2
Python: 3.3.4
Arch Linux