We describe a case of a right-handed, 42-year-old, Caucasian female patient who manifested a set of unusual adverse effects during maintenance ultrabrief right-unilateral electroconvulsive therapy (ECT)-generalized myoclonic secondary seizure with lateralization nonresponsive to repeated intravenous diazepam application, deviation of the tongue and the head to the left, ping-pong gaze with nystagmoid jerks, postictal hypoactive confusion state lasting 15 minutes (with a total time to recovery 30 minutes), and likely Todd's paralysis after the procedure (subsided within 24 hours). These adverse effects led to a thorough clinical investigation and eventually the discovery of a brain tumor. In the article, we hypothesize about the possible interaction between the intracranial mass and ECT and provide a literature overview on the topic. Cases like this are likely underreported, and although several studies demonstrate that ECT can be applied safely to patients with an intracranial mass, this report demonstrates that brain tumor can interfere with ECT in an unpredictable way and have severe consequences (eg, unresponsiveness to diazepam when attempting to halt the seizure as our patient). Unusual symptoms after ECT, albeit reversible, such as in this case report, should be followed by a thorough neurological check-up to exclude any underlying organic pathology.