The Essential Repair Kit
Pack this in your convention bag: safety pins (multiple sizes), needle and thread in your primary costume colours, body tape or fabric tape, contact cement in a small container, spare wig pins, and a small pair of scissors or a seam ripper. This kit weighs almost nothing and solves the majority of convention costume failures.
Common Failures and Fixes
Split seam: safety pin until you can get to a quiet space; then hand-stitch closed with the needle and thread from your kit. Broken zip: safety pins used decoratively as faux closure if the zip is on the back; body tape if it's a front opening. Detached armour piece: contact cement, pressed for two minutes. If you have hot glue and a mini gun, use that. Wig slipping: wig pins in the wig cap; check they're going through both the cap and your hair. Hem dropping: body tape applied to the inside of the hem allowance; press firmly. Holds for a full day.