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But in theory even the simple circuit which muxes two inputs onto one output could glitch when the output selection tuns off faster than the other selection turns on.
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Very true and that is what is worrying me.
The problem seems to occur when two LEs are required to contain the code rather than one. If this occurs then it may be an output from one LUT which has to turn on/off at the same time as a LUT in another LE. Due to the delays that this may cause, glitches result. If the code fits exactly into one LUT then no glitches result as it is designed to be glitch free.
To try an understand this a bit more I have tried to simplify the design to the minimum that causes a glitch and came up with the following:
assign q = s & a
| s & b
| !s & c
| !s & d;
With a,b,c,d set to 1, toggling s causes a glitch. If you remove one of the inputs, so it has 4 rather than 5, it will fit into one LE and no glitches result.
I think this explains it so I can now design around it.
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