It was indeed the 40th anniversary issue that I was referring to. It included many different strips drawn by various artists and on this occasion the characters met Tintin.
Another example was another 2-pager of the series "Jonathan" by Cosey, pen-name of Bernard Cosendey. Jonathan's adventures take place mainly in the Far East and he is often based in India or Tibet. In this 2-pager, Jonathan is wandering through the snows of Tibet when he meets Tintin and Haddock.
Haddock tells Jonathan that he can join them provided he does not have a radio with him! Tintin explains that the captain feels that Tibet is one of the last places on Earth where they can avoid Castafiore (a reference to the radio incident on page 17 of "Tibet").
They discuss the "Calculus Affair" (notably the scenes in Cosey's native Switzerland). While they chat, the yeti discreetly pinches a bottle of whisky and is half-way up the mountain by the time Haddock realises it's gone.
More details about "Jonathan" can be found at Cosey's entry on the Lambiek website
, the website of a leading comic shop and thus quite legitimate.
Curiously, one strip "Clifton", drawn by Bob de Groot, had the character, an English detective, meet E. P. Jacobs, who even paints the sign of the "Yellow M" on a wall! It was probably raising the fact that artists other than Herge also contributed to the success of the magazine.