When using method=crossCollection in a Join query, if a document does not have the field used for the from param, a NullPointerException is thrown.
This is pretty simple to reproduce using the techproducts sample data (following assumes a clean install):
Depending on which shard the error comes from, you might see a "No live SolrServers available to handle this request" error, but digging into the logs for the other shard that served the query will show the same NPE in the example above (I'll put a full stack trace into a comment).
"Regular" (non-cross-collection) joins work fine if some documents are missing the field.
If we shouldn't expect XCJF to work when documents are missing the from field, then we should document that as a limitation and also throw a more descriptive error.