Large key sizes (larger than 1024 bits).

Most newer versions of Stronghold warn you that making a key greater than 1024 bits will result in browser errors. However, there are some that don't and it is possible to create a 4096 bit key and CSR (and thus certificate). 4096 bits is very secure, but it is slow and most browsers don't handle them particularly well. You'll have to ask your representative about the $50 re-issue so you can regenerate your key (to 1024 bits this time) and submit a new CSR for signing.