If a charged creeper kills a stray skeleton type, it will always drop a wither skeleton skull rather than an ordinary skeleton skull or nothing. Many strays can be killed at once to obtain many wither skulls. If done in survival, a player with all of these wither skulls could spawn large amounts of withers too.

Found this bug when experimenting with channeling tridents in creative.

Relates to:
https://bugs.mojang.com/browse/MCPE-35876