Ugh, that's the worst feeling. Honestly, I got burned like that once and now I just take a super thin pick and gently work it into the crack to feel for resistance. If it slides in way too easy, I know I'm in for a long day.
Watched my buddy do that exact thing last summer on an old porch step. He got the thinnest metal ruler he could find and slid it in like you said. Thing went in a full three inches before it even slowed down. We ended up replacing the whole stringer.