$path = 'C:\text.txt'
$output = 'This is an output string'

# This works...
[System.IO.File]::WriteAllLines($path, $output)

# But this doesn't. WTF!
[System.IO.File]::AppendAllLines($path, $output)
# Result: 'Cannot find an overload for "AppendAllLines" and the argument count: "2".'

# However, if you cast the string to an array, the correct overload can be found:
[System.IO.File]::AppendAllLines([string]$path, [string[]]$output)

# Hooray!