This week's fascinating new video Trickdiebe am Frankfurter Flughafen uses a slang expression that is potentially confusing, in that its use of nicht would seem to contradict what the expression actually means.
Beim Präventionstag staunt so mancher nicht schlecht, worauf man alles achten muss.
The first impression might be, since the negation nicht is in the sentence, that some people are not amazed. But in fact, the phrase nicht schlecht staunen in this context means nicht wenig staunen: "not a little amazed" or "quite amazed."
On crime prevention day some are quite amazed at all the things you have to watch for.
Captions 7-8, Trickdiebe am Frankfurter Flughafen: Die Bundespolizei klärt auf
And another example on Yabla German:
Sie zog den Vorhang beiseite und staunte nicht schlecht.
She pulled the curtain aside and was quite amazed.
Captions 41-42, Märchen, Sagenhaft: Das kleine Mädchen mit den Schwefelhölzern
This is the only example that I am aware of in German where the phrase nicht schlecht is used in a possibly confusing way. Even the meaning of the slang phrase nicht schlecht, Herr Specht is pretty obvious in context: "Well done!" The phrase can also be used ironically if somebody has made a mistake or performed badly. Herr Specht probably does not refer to der Specht (woodpecker) here, it is rather just a rhyming word that adds emphasis to the phrase, kind of along the lines of the English catchphrase "no way, José!"
The verb staunen also has some other slang or idiomatic phrases associated with it, such as Bauklötze staunen (be very surprised) and aus dem Staunen nicht herauskommen (not cease to be amazed).