One of the most dangerous times is not IN the thermal, but getting
there. When you're focused intently on the 3 gliders at the same level
you want to join, it's easy to miss the guy who's ALSO approaching the
same thermal from a slightly different angle. I've got a nice near
miss [as opposed to a bad near miss] in the flight logs from a
nationals where a high-time professional pilot type slid smoothly into
a crowded thermal...and completely missed me coming in from an angle
about 30 degrees off his. That the "miss" was literal and not just
figurative was because I saw him coming and pulled up and over him as
he bored into the thermal like a laser beam, merging neatly with the
crowd there.

Keep scanning, even--especially--as you get close to the thermal.
There's a pretty good chance that anyone else approaching the thermal
at the same time is focused intently on the gaggle, not on the gliders
coming into it. This is especially true locally and pre-start when
gliders are milling about but the incident I mentioned occurred on
course when one pilot found a good thermal close to a gaggle and
multiple gliders converged on him from several directions.

Chip Bearden
ASW 24 "JB"
USA