Although OS-level details of files vary, in general, when you have a file open in r+ mode and do some read or write operation, the “current position” is left after the last read or write.

When you did:

fileContents = erroredFile.read()

the stream erroredFile was read to the end, so the current position is now “at the end”.

The truncate function defaults to using the current position as the size to which to truncate. Assume the file is 100 bytes long, so that the current position “at the end” is byte 100. Then:

erroredFile.truncate()

means “make the file 100 bytes long”—which it already is.

The current position remains at the end of the file, so the subsequent write appends.

Presumably you wanted to seek back to the beginning of the file, and/or use truncate(0) (note that just truncate(0) will, at least on Unix-like systems, leave the seek position at the end of the file so that the next write leaves a hole where the original data used to be). You could also be slightly more clever: if you’re inserting, just overwrite-and-extend in place (no truncate is required at all).

(Joel Hinz already answered the second question, I see.)

Not a Python answer but it may widen your horizon. Use sed:

$ cat input.txt 
foo
bar
baz
INSERT HERE
qux
quux

$ sed '/INSERT HERE/anew stuff' < input.txt
foo
bar
baz
INSERT HERE
new stuff
qux
quux

The command a will append the text on a new line. If you want to insert the text before the match, use the command i:

$ sed '/INSERT HERE/inew stuff' < input.txt
foo
bar
baz
new stuff
INSERT HERE
qux
quux

Why do you not try a two steps solution? At first, you read and fix the string, at the second step, you rewrite the file. Probably it’s not the most efficient algorithm, but I think it works.

with open(erroredFilepath, 'r') as erroredFile:
    fileContents = erroredFile.read()   
    fileContents.replace('insert_here', 'insert_string')

with open(erroredFilePath, 'w') as fixingFile:
    fixingFile.write(fileContents)

test.txt

a
b
c
d
e

1. Read into a list then overwrite

def match_then_insert(filename, match, content):
    lines = open(filename).read().splitlines()
    index = lines.index(match)
    lines.insert(index, content)
    open(filename, mode='w').write('n'.join(lines))


match_then_insert('test.txt', match='c', content='123')

Result

a
b
123
c
d
e

2. FileInput

from fileinput import FileInput


def match_then_insert(filename, match, content):
    for line in FileInput(filename, inplace=True):
        if match in line:
            line = content + 'n' + line
        print(line, end='')  # Redirect to the original file


match_then_insert('test.txt', match='c', content='123')

3. seek

def match_then_insert(filename, match, content):
    with open(filename, mode='rb+') as f:
        while True:
            try:
                line = f.readline()
            except IndexError:
                break
            line_str = line.decode().splitlines()[0]
            if line_str == match:
                f.seek(-len(line), 1)
                rest = f.read()
                f.seek(-len(rest), 1)
                f.truncate()
                content = content + 'n'
                f.write(content.encode())
                f.write(rest)
                break


match_then_insert('test.txt', match='c', content='123')

Compare

from timeit import timeit
from fileinput import FileInput


def init_txt():
    open('test.txt', mode='w').write('n'.join(['a', 'b', 'c', 'd', 'e']))


def f1(filename='test.txt', match='c', content='123'):
    lines = open(filename).read().splitlines()
    index = lines.index(match)
    lines.insert(index, content)
    open(filename, mode='w').write('n'.join(lines))


def f2(filename='test.txt', match='c', content='123'):
    for line in FileInput(filename, inplace=True):
        if match in line:
            line = content + 'n' + line
        print(line, end='')


def f3(filename='test.txt', match='c', content='123'):
    with open(filename, mode='rb+') as f:
        while True:
            try:
                line = f.readline()
            except IndexError:
                break
            line_str = line.decode().splitlines()[0]
            if line_str == match:
                f.seek(-len(line), 1)
                rest = f.read()
                f.seek(-len(rest), 1)
                f.truncate()
                content = content + 'n'
                f.write(content.encode())
                f.write(rest)
                break


init_txt()
print(timeit(f1, number=1000))
init_txt()
print(timeit(f2, number=1000))
init_txt()
print(timeit(f3, number=1000))