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!