Reproducing code example:

import numpy as np
y = np.convolve(np.ones(1,), np.ones((1,))/1, mode='random_mode')

Error message:

637 def _mode_from_name(mode):                                  <--- numpy/core/numeric.py
638     if isinstance(mode, basestring):
639         return _mode_from_name_dict[mode.lower()[0]]            
640     return mode

KeyError: 'r'

NumPy/Python version information:

1.18.5 3.7.6 (default, Jan 8 2020, 19:59:22)
[GCC 7.3.0]

Discussion:

Current modes for numpy.convolve, and numpy.correlate are 'valid', 'same' or 'full'. If there is any other string provided it raises a KeyError with the first letter.
In fact,

import numpy as np
y = np.convolve(np.ones(1,), np.ones((1,))/1, mode='very_random_mode')

Does NOT raise an error. As it is simply mistaken for 'valid' (the very first character is checked)
Think there should be a check here: