In "cooker.py" there is a section of code that scans a layer for its priority.

However, only the success path avoids an otherwise undefined variable, so the error is not reported and bitbake crashes. This was found while experimenting with a weak layer.
  
  UnboundLocalError: local variable 'cre' referenced before assignment

Here is the code in question. Notice that 'cre' is only defined on the success path.

vvvvvvv
# Calculate all layer priorities using calc_layer_priority and store in bbfile_config_priorities
for c in collection_list:
    calc_layer_priority(c)
    regex = self.data.getVar("BBFILE_PATTERN_%s" % c)
    if regex == None:
        parselog.error("BBFILE_PATTERN_%s not defined" % c)
        errors = True
        continue
    elif regex == "":
        parselog.debug(1, "BBFILE_PATTERN_%s is empty" % c)
        errors = False
    else:
        try:
            cre = re.compile(regex)
        except re.error:
            parselog.error("BBFILE_PATTERN_%s \"%s\" is not a valid regular expression", c, regex)
            errors = True
            continue
    self.bbfile_config_priorities.append((c, regex, cre, collection_priorities[c]))
^^^^^^^^

I propose that the resolution is to move the append right after 'cre' is defined, since that is the only success path.