I would assume at some point, too many uncorrected mistakes during the shemone esri (not talking about the things that would make it avodah zara here) would make it invalid/useless as it simply loses its meaning (literally).
At what point does this happen? Do only the brachot have the be intact? Do even those have to be intact? Should one use the same standards that they do for reading the torah? What if the sheliah tzibur makes the mistakes?