I just found an good example of an ambiguous ὃτι clause: Luke 1:45. It comes as Elizabeth blesses Mary. The Greek goes:
καὶ μακαρία ἡ πιστεύσασα ὅτι ἔσται τελείωσις τοῖς λελαλημένοις αὐτῇ παρὰ κυρίου.
THe question is, is that ὃτι nominal or causal?
A nominal (or content) ὃτι clause is one which “introduces a subject, predicate nominative, direct object, or an appositional noun clause” (Wallace, 678). Such clauses are usually translated “that…”.
A causal clause is one which “expresses the basis or ground of an action” (Wallace 674).
Both interpretations fit well in this context. If it is a causal clause, then Elizabeth is saying that Mary is blessed because of what God will do to her. Alternatively, a nominal clause here would give further details about what it is that Mary has believed. Bock (139) give a good summary of the arguments either way.
So how does one decide which it is?
Firstly, given that either is possible and there are reliable scholars on both sides, we have to admit that we can’t be too dogmatic about it.
Secondly, check the versions. NIV, ESV and NET all go with nominal. KJV, NKJV, HCSB favour a causal clause.
So far we’re still evenly balanced. The thing that sways it for me is the immediate context. If the clause is causal, it modifies the omitted ἐστίν (“Blessed is…because…”). If it is nominal, it goes with the immediately preceding πιστεύω (“has believed that…”). When we look up πιστεύω in BDAG (p816) we see (under 1. a. β) that an ὃτι clause with πιστεύω is a very common usage (BDAG even cites this verse as an example).
I would conclude that since we have in the immediate context (i.e. the preceding word) a valid explanation and interpretation for the ὃτι, then there is no need to look to the wider context of the rest of the verse (i.e. the earlier “Blessed [is]…”).
That’s my initial thought, anyway. Feedback welcome.