There are a few that I can think of - mainly used by engineers trains, but occasional out of service workings of passenger stock heading to Acton Works.
At Finsbury Park there is a connection between the Victoria and Piccadilly lines. This cannot be used by the 2009 stock, as it is too wide for the Piccadilly.
At King's Cross St Pancras, there is a connection between the southbound Northern line and the Piccadilly line.
At Baker Street there are connections between the Jubilee and Bakerloo lines, a legacy of when the Stanmore branch was part of the Bakerloo.
Out of the tunnel, there is a reverse connection at West Ruislip depot between the Central and Metropolitan lines east of Ickenham station.