When writing new advancements or recipes and a parsing error occurs and the error "Expected item to be a JsonObject" is thrown, the details of that error are heavily truncated to the point of uselessness:

[13:56:44] [Server thread/ERROR] [FML]: Parsing error loading built-in advancement harderfarming:recipes/thermometer
com.google.gson.JsonSyntaxException: Expected item to be a JsonObject, was "min...op"

What on earth is "min...op"? This makes it hard to locate the error in my JSON file.

This is due to the following code located in JsonUtils.toString:
StringUtils.abbreviateMiddle(String.valueOf((Object)json), "...", 10)
The "10" indicates how long or short the displayed string should be, in this case, 10 characters, of which the "..." counts for 3. The starting and ending quotes then count for 2. That leaves 5 characters from the actual JSON file that can be displayed. This is not nearly enough.

Suggest raising the value to 15 (to account for the 5 characters that are forced) or 20 (to be less aggressive generally). That would result in:
(at 15): "minec...kchop"
(at 20): "minecraft:porkchop"

15 would be sufficient to locate the error, although difficult to read, while 20 would be sufficient to show the entire string for a majority of items, but also not increase the size of the error message unduly for lengthy issues.