I think some UCI engine does not handle correctly UCI protocol in some special circumstances, among them Stockfish (and of course some Glaurung versions, I did not check all).
For example with Stockfish 1.6, enter the following commands :
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uci
position fen 7R/p5p1/1p4p1/n1pq2k1/6Q1/P2r3P/6P1/4R2K b - - 6 39
go infinite
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info depth D
info depth D score mate -2 time 129 nodes 10154 nps 78713 pv g5f6 h8f8 d5f7 g4f4
Then just after this, if you enter
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stop
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Unknown command: stop
And in the UCI protocol it is said that after a "go infinite" :* stop
stop calculating as soon as possible,
don't forget the "bestmove" and possibly the "ponder" token when finishing the search
This is what makes Stockfish feel frozen under Scid (it may affect other interfaces, I guess).search until the "stop" command. Do not exit the search without being told so in this mode!
Having said that, I have to say those engines are among the best in the world . Congratulations to the authors !
Pascal Georges