Last night just before leaving home I've tuned my bass in order to 'save time' at church where we're practicing for a play coming up in a fortnight. We did not follow the usual pattern of first 'setting up' the band and a few seconds into the first song I realised that something's very wrong! I checked my bass' tuning and ALL of my strings were way too flat!
The band went ahead full-steam and I had to improvise in order to tune (placed the tuner right in the bass amp's face) - no opportunity to unplug the cable & plug in the tuner as the soundman had his own problems in setting up the vocalists (now that I'm thinking of it - I should check whether the amp has a line out and plug in the tuner in advance every time we're playing)!
Lesson 1 learned: Always leave your bass for a few minutes at the venue where you'll be playing (to adjust to temperature differences) before tuning!
Lesson 2: Always insist on a proper setting-up session before playing the first song!