Undisclosed caffeine dose
If caffeine is buried in a proprietary blend, you don't know if you're getting 100 mg or 400 mg. That's a cardiovascular gamble.
Yohimbine + high caffeine
Both are potent stimulants that act on different receptors. Stacked in one scoop, they've caused ER visits. Avoid.
'DMHA', 'DMBA', '1,3-DMAA'
Amphetamine-like stimulants that have been banned repeatedly and keep getting relabeled. Any product with these is a hard skip.
'Nootropic' blends at trace doses
L-tyrosine, alpha-GPC, huperzine — all real ingredients, all typically underdosed to the point of doing nothing.