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Clutch pedal wasn't depressed enough. Could be many reasons why; seat too far back, thinner soles on your shoes thus rendering muscle memory somewhat redundant, folded carpit.
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Tried to put it into reverse. This is dependent on the gearbox though, some wont let you anywhere near reverse without some sort or purposeful action. Some will let you partially slot it into reverse, and thus you'll get a grinding noise (A lot of boxes in Fords do this).
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Mechanical problem.