In your jersey back pockets: tube folded tight with a rubberband to hold it. power link (sram link, connex link - whatever you use)for your chain. multi tool, tire levers (little plastic ones)
Taped to you seat post: CO2 dispenser. Some people tape the power link to their handlebars.
Those under the seat packs tend to unzip themselves & scatter your stuff all over the trail....
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As with mud, life, too, slides by.