Ruby: Equal Plus Not Equal to Plus Equal

I was recency debugging an issue on an app. I had my test setup and properly failing. I was debugging and narrowing it down to the correct file. I wasn't getting an error or exception, so the specific line number wasn't known. I just knew where the general error was.
I scanned the file, only about 15 lines. I was looping through to some form fields and needed a counter like below:
- if job_skill.id.blank? - key = "key#{id}" - id =+ 1
If you paid attention to the title or have a sharp eye, you might have noticed the slight difference:
=+ != +=
I would have thought that =+
would cause some sort of exception, but
it doesn't:
[1] pry(main)> i = 0 => 0 [2] pry(main)> i =+ 1 => 1 [3] pry(main)> i =+ 1 => 1 [4] pry(main)> i =+ 1 => 1 [5] pry(main)> i += 1 => 2 [6] pry(main)> i += 1 => 3 [7] pry(main)> i += 1 => 4 [8] pry(main)> i += 1 => 5 [9] pry(main)>
So, as you can see, my first two results were as expected, but then all future results overwrote the 2nd.
Nice little gotcha to watch out for!