

Portrait shot of brewer in Isle of Man, first shot, flash fires but strobe warning beep goes off that it's not been able to put out enough light. LCD on back shows shot massively underexposed.
I'll quickly explain here that while charging everything up for the trip, began to think about how long I've had my rechargable AAs and wondered if they had a limit on the number of charge cycles you can put them through, which may explain my getting locked into the wrong solution.
Immediately assumed it must be batteries, so change, do a test shot of a nearby wall, everything looks fine. Another shot of subject, another underexposed shot and warning beep.
Bit embarrassed now, decide perhaps I'm right about batteries not holding charge and put another fresh set of rechargeables into the flash.
Brief moment of clear thinking and I do actually think about checking settings - notice camera is still in sports mode from when I last used it at the weekend. Shutter priority, auto ISO, so everything all over the place and while I've got one half usable shot (bottom of two posted), I can even see on the LCD it's too noisy to print. Reset to "portrait" settings bank, thinking this will solve issue.
Another underexposed shot and with subject clearly losing patience, let him escape. Try again 10 minutes later against different background with another set of fresh batteries, same bad results (first of two posted). By this time he thinks I'm some fool who's been handed a camera for the first time - so I give up.
Only when I've got back into the car and have another, slightly calmer look at the camera that I realise while I've switched the camera from sports to portrait settings and from shutter priority to manual (I usually shot indoor flash shots at around f8, 1/30s) the shutter speed from the weekend shooting remained the same when I switched to manual (1/500).
Lessons learnt/relearnt:
I always used to set my camera back to a "default" setting before putting it away - which might have helped, except logically it might be best to store camera set up for quick shooting/action shots. Perhaps should start doing this again!
Also always used to do the WIFE check before shooting (WB, ISO, focus, exposure) but got a bit complacent.