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Signs & symbols
Glyph
Alchemy
Alchemy, problem, spiritual
Black
512x512
Mixed
Free Misc Icon Set 2
Alert, problem, exclamation
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Simple darkcon
warning, Error, skull
324x512
problem, warning, Caution
UI
Very Basic. Android L (Lollipop)
problem, Attention, warning
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Business & finance
24x24 Free Application Icons
problem
24x24
Attack, Error, Alert
DarkSlateGray
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Desktop apps
16x16 Free Toolbar Icons
problem, warn, danger
Gold
16x16