An elderly woman presented with abdominal pain.  Here is her ECG:

This is easily mistaken for atrial flutter because the waves have a fluttering, wavy, sawtooth appearance.  However, 3 things make atrial flutter impossible, 2 are related:

1) the rate of the waves is > 400

2) the QRS does not appear at the same part of the atrial wave cycle each time (which it must do in flutter)

3) the ventricular rate is irregularly irregular.  In atrial flutter, the ventricular rate can be regular if there is a constant ratio (2:1 block, 3:1 block etc).  It can also be regularly irregular, in which every QRS comes at a multiple of the atrial wave rate (if atrial wave rate is 300, it happens every 200 ms, and every R-R interval must be a multiple of 200ms)

2) and 3) are related: since 2) is true, 3) must also be true.

The woman had atrial fibrillation.  It was misdiagnosed as flutter, and this may have contributed to a delayed diagnosis of mesenteric embolism.