I have run into an example failing with Unrecognized base in input sequence, without being told what the problematic base is.
Quoting src/libprimer3/libprimer3.c this warning/error message does not but could easily report the offending character:
if ((offending_char = dna_to_upper(sa->trimmed_seq, 0))) {
if (pa->liberal_base) {
pr_append_new_chunk(warning,
"Unrecognized base in input sequence");
} else {
pr_append_new_chunk(nonfatal_err,
"Unrecognized base in input sequence");
return 1;
}
}