Read the label on your ammo before putting it in your bag/box. For my last club bullseye match, I grabbed a rectangular plastic box that said CCI on it. When I went to load my first magazine, my first thought was "I don't remember Standard Velocity being jacketed...?". Then I realized they were Mini-Mags instead of SV. Wouldn't even feed in my S&W 41.
Luckily the guy in the next lane was nice enough to lend me a box of SV, but it still didn't help my composure in the first string of slow fire.