Over time, the "stuff" from your hands and residue on the outside of your NMR tubes builds up on the inside of the inserts inside the coil of the NMR probe. This "stuff" contains protons and results in a background signal in all subsequent NMR spectra. I have seen inserts of NMR probes so dirty that samples have "stuck" inside the probe. The offending gunk resembles grey-black bubble gum or perhaps dirty dried nasal effluent. Periodically NMR probes must be cleaned to remove this offensive residue. This can usually be accomplished by gently inserting and removing a cotton swab soaked in alcohol inside the coil insert. For the particular case in the figure above, the NMR probe could not be cleaned in this way and had to be disassembled by a service engineer and cleaned in an ultrasonic bath.
WIPE YOUR NMR TUBES BEFORE PUTTING THEM IN AN NMR PROBE !!