There doesn't seem to be a full-length (or even short) study on this fascinating topic: if there is/are, please post your recommendations.
     I've always wondered who (which opera house) sanctioned the first operatic cut, and why. If the brief Scarpia-Tosca exchange after 'Vissi d'arte' precludes applause on the stage (what an excuse!), why cut it in the studio? The cut of a second verse or an entire aria is even more infuriating: I personally insist on BOTH verses of Leonora's 'Di tale amor' AND 'Tu vedrai'. The most annoying are those tiny cuts which save only a few seconds, as well as the deletion of lines sung by individual bystanders or the chorus (eg Violetta's death and Lucia's mad scene). Incidentally, the only cut which I think is an improvement (though I still object to it!) is that of the concluding bars of ANNA BOLENA: having heard the Callas studio mad scene from this opera, I find the complete version less effective as a conclusion to the work. Why does the ROH include every single bar in their 15-hour long RING but make cuts in their TROVATORE (only 140 minutes when performed complete)?
     Who is to blame: the management, director/producer (no time to debate the exact appellation now!), conductor, singer? How did composers react to such 'unauthorized' cuts during their lifetime?
     Any thoughts on cuts and revisions? (I'll post mine on the latter subject soon, citing various recordings of DON CARLO as a unique example: I definitely need IGI's help here!)

Cuts and interpolations.  Composers authorise cuts and alterations.  Apparently, Wagner sanctioned an immensely long cut in "Rienzi".  There are two endings to Gounod's "Mireille", an opera that contains some of his most attractive music.   Madame Butterfly was revised and amended several times with authorised cuts and alterations.  I'm not too sure if the opera we hear today is actually what Puccini finally wanted.   Rossini, of course, got completely fed up with the abundant florid decorations singers used in his early operas.  I don't know if he approved of a soprano Rosina with interpolated high notes. On occcasions, all operas have cuts imposed - by the singers, the conductor, the producer and  - time/cost. 

As someone who has written one and had it produced, I think s'wave is right. And I have to say, I found the experience of being asked to cut, dovetail, swap around, rewrite, write more etc. etc. very illuminating! I was aksed to do these things by both director and conductor, often at rehearsals and on the hoof, as it were. There was a perfectly good, often very practical, reason in each case. If the opera was ever produced again, I would expect to go through the process again - I don't see how, with such a complex and collaborative medium as opera such things can be avoided.
Plus, I think all opera composers who understand the realities of theatre should stand ready to do this kind of thing. (Mozart, Verdi, Britten... they all came to rehearsals with extra MS paper, didn't they!  )

Thank you all for your replies. Your PARSIFAL story is interesting, Tony. I confess I didn't know of Humperdinck's Wagnerian contribution: does this bit exist?

 I was listening to the Hickcox recording of Trial by Jury.  I noticed a change to probably the best known line.  The Macmillan libretto gives:

She could very well pass for forty three
In the dusk with the light behind her.

Instead Donald Maxwell sang something along the lines of

She often is taken for forty three
In the dusk...

I suspect this is another example of using the original version when the librettist changed it for the better in subsequent revivals.

Perhaps the Savoyard of Oswestry can comment.

Tony.  It does scan better and makes more sense.  "Could very well pass" really means she could never pass.  The exact age, 43, makes the whole phrase more malicious. 

Any more recent recordings of Trial other than with a two act opera?

I was struck listening to the work how utterly cynical all the characters are, even the Usher ("What he may say you needn't mind..."

I would be gobsmacked if Britten was thinking of the pompous little theme that introduces the Usher when he wrote the comparable music for Swallow at the start of the inquest in Grimes, but there is a comparison.

I should think operas have been cut and revised since day one,and the practice probably came from the spoken theatre where it was already common.Certainly Handel and Mozart were quite used to singers and directors demanding   substitute arias,etc.

I was very glad when Paul Daniel and Opera North restiored the original and complete 'King Priam'.I was never  haopy with the Covent Garden cuts and changes to the libretto.

I'm probably one of a very few who believe Tippett knew exactly what he was doing in his libretti, and as with  Bruckner's symphonies, I believe his original wishes will one day be respected.

One instance stands out.When Paris says ' I will .. go and found a new Troy',Priam replies 'You are not that kind of man'. But in the original his reply is much more cutting : 'You are not the founding sort'.

smittims, don't tell me we actually agree on something for once!