I showed up on one and didn't even know it in Baltimore one time. My partner and I turned down an alley off of North because we saw a large crowd gathered in it and just wanted to see what they were doing. When I drove up into it, there was a lone female officer trying to handcuff a resisting subject she had chased into the alley, with about fifty "citizens" watching and rooting for the subject.
About the time we got the cuffs on him, I started hearing about thirty different sirens and I asked the officer if she had called in a Signal 13 (I didn't have my BPD handheld with me that day). She had, and it took us about twenty minutes to get out of that alley after both ends got filled up with units.