Throttle friction nuts
Having personally watched this happen from the cabin of a 200, and having to run to the front to correct the impending disaster, I will throw his into the fray as its a serious issue if your not fully in control and flying in marginal to bad conditions.
The slow deteriorating performance along with the confusion of the instruments can lead the pilots to miss the actual problem. Read the report to understand.
https://assets.digital.cabinet-office.gov.uk/media/5422ebafe5274a13170000a7/dft_avsafety_pdf_022814.pdf