This sounds like a focus calibration issue: either front focus or, from your description, more likely back focus. In that case the AF system confirms focus, but the sharpest plane ends up in front of or behind the selected AF point.

A simple way to test it is with a focus chart or ruler test: place a target with a clear center mark, shoot it at a slight angle, use the center AF point, and use the lens wide open (lowest f-number) so depth of field is shallow. Then check whether the sharpest area falls on the target line or consistently behind/in front of it.

If the problem repeats consistently, the camera and lens likely need calibration. On an entry-level body like the Rebel T1i, user AF microadjustment isn’t available, so the practical fix is service calibration through Canon or an authorized repair center. If you have multiple lenses, testing them can help show whether the issue is one lens or the body/lens combination.