I'd never used a router before and did a couple of test cuts on some spare MDF, these cut perfectly with the slot nice and central, however on cutting my first cabinet side, the cut initially started off ok and then somehow went adrift - user error

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I though about filling but found the slot too narrow to get a good amount of filler in, so I have now recut all the slots correctly, obviously some parts are now way to wide for the T moulding, so my option are, now it's wider I can get filler in and then recut again when dry, or I can glue the T Moulding where the slot is too wide!
Any helpful comments please
