I added optifine and to Tekkit Lite, and then it began acting... weird. No crashes or anything, but certain blocks and items do not update unless you either break them or right click them.
Just a list of what happens:
Breaking a door or a bed breaks only the block you broke, leaving the other half still there (drops the door/bed anyway)
Random chance for trees when broken for the whole tree to remain there (but the items drop, and the tree remaining gives nothing extra nor updates)
Removing or adding water/lava blocks will not update the "current" of the water/lava (e.g. breaking a block next to a water block will not show the water moving there, but you can still be dragged by the current, drown or set fire with lava)
breaking a grass block with tall grass on it will leave the tall grass there (will still drop seeds)
Random chance for dirt blocks to not turn into grass blocks outside.
All of these issues are mostly minor (though hidden lava can give you a bit of a surprise), and are solvable by right clicking with an empty hand or something that can't be broken or placed (although sword blocking a glitchy part is fine), but seeing as my cruddy ol' laptop needs optifine to run MC smoothly, I'll just stick to Tekkit.