Without verbalized intent, how would we know the korban was invalid? Or just need to rely on the kohen to admit the mistake afterwards? Were the people bringing korbanos close enough to the offering to be able to hear what the kohen was saying?
Joshua Danziger
1) Tosfos Zevachim 2a (end of paragraph beginning Kol) writes that in Tosfos Bava Metzia 43b (paragraph beginning HaChoshev) he explained whether the "thought" that invalidates kodashim means that it is only if one spoke and said one has the wrong intent that the korban is invalid; or does it mean literally the wrong thought.
If we look up Tosfos Bava Metzia 43b we see that Tosfos writes that one has to say one has the wrong intent and it is not sufficient merely to think about it.
Tosfos Zevachim top 4b DH Machshava comments on the Gemara that uses the word "thought" and writes "even if one says that one requires speech". This implies that it is not so clear that one must actually say the wrong thing to invalidate the korban because of the wrong thought. However Tosfos seems to be more inclined to the opinion that it is not sufficient merely to think it.
2) The Mishnah Tractate Keilim 1:8 states that a Yisrael may not enter the courtyard of the Kohanim unless they want to do Semichah; leaning on the korban; or to slaughter the korban (a Yisrael may slaughter the korban) or to wave the korban. However for these 3 purposes the owner of the korban is allowed to get close. So we see that sometimes the people could be close enough to the kohen to hear what he is saying.
We should be zocheh soon to bring korbanos in the Beis Hamikdash!
KOL TUV
Dovid Bloom