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A subspace anomaly is a term for any sort of disruption to subspace. These anomalies can often be hazardous for starships and people alike.
While some anomalies were naturally occurring it was possible for them to be artificially created, particularly through the use of subspace weapons. (Star Trek: Insurrection; Star Trek: Swiftfire: "Regression")
The Dominion lured Terri Letac off the USS Swiftfire-A in early 2375 with a false invitation to a conference on subspace anomalies. (Star Trek: Swiftfire: "Always on My Mind")
Subspace anomaly article at Memory Alpha, the canon Star Trek wiki.