Revealing mistake: When Keyface is leading Imogen and Melissa on chains, he hits Elise, sending her through the air. When this happens, you can pretty easily see that Elise is played by a stunt-woman. You get a very quick look at her (suddenly much younger) face, and when she lands, you can see that her body is also has a slightly thicker, more toned build than Elise's actress Lin Shaye. (Slow motion helps but is not required).
Revealing mistake: Normally I wouldn't consider a bad CGI effect to be a mistake, but when Elise finds the hidden keyhole in the wall in the basement, the effect is comical if you pay attention. The way it appears as she scratches is pretty unnatural, and the pieces of paint that fall practically look like cartoon pieces if you pay attention. The shadow of her left hand also looks a little too sharp compared to the rest of her (somewhat softer) shadow, revealing it was likely rotoscopped in some way.
