When speakers or performers roam with mics, feedback frequencies may shift. Always try to perform stability testing in a manner that resembles real world use. An otherwise stable system may lose stability when mulitple mics are open simultaneously. In this case, the best solution might be to train the operator to keep microphones open only when they are use. Educating users in mic technique can also help.
Many people tend to grab mics or lean very close when they speak.
Both of these actions can cause problems. Grabbing a cardioid microphone can increase its physical gain at certain frequencies when the user's hand closes off the rear ports to the microphone element, making a stable system suddenly unstable. When people lean too close to a mic, they can reflect some energy at problem frequencies back into the mic, possibly causing feedback. Some reverberation units can cause an otherwise stable system to become unstable. With a simple enough system, polarity or phase changes could solve a feedback problem immediately.