A 65 year old presented with altered mental status and had an intracranial bleed:

One could be fooled into thinking this is sinus tachycardia (with a short PR interval) with diffuse ST depression.  But close inspection reveals flutter waves.  In particular, a totally upright p-wave in V1 is very unusual and should alert you to atrial flutter.  The fluttering baseline accounts for the apparent ST depression, although I cannot rule out some amount of true ischemic ST depression.

 Here is the ECG after cardioversion:

Now there is sinus.  Interestingly, this one also has an upright p-wave in V1 – so the rule is not universal!