LRH described in detail the harm caused to a case by overruns:

“In auditing, an overrun means the preclear came out of the bank and then went back into it again … By continuing, the auditor throws the pc back into the bank again and wrecks the release state.”

HCOB 19 December 1980R
Rehab Tech
(Tech Vol. XII, p. 202)

“An 'overrun' means doing something too long that has engrams connected with it…

If this overrun persisted unhandled eventually the pc would be overwhelmed …”

HCOB 16 June 1970
C/S Series 6, What the C/S is Doing
(Tech Vol. IX, p. 104)

“… An OVERRUN is full of MASS and ARC breaks!”

HCOB 3 June 1971
C/S Series 37R, High and Low TA Breakthrough
(Tech Vol. IX, p. 354)

“Auditing a pc past a state of Release on the processes of that stage can make the pc’s tone arm rise and bar further case gain even at upper stages of Release.”

HCOB 6 August 1965
Qualifications Technical Actions
(Tech Vol. VII, p. 639)

As far as why anyone would overrun pcs in the first place:

“The first goof relating to Releases is the one done for fifteen years — running past a free, floating needle on any type of process. THIS is the goof that held back all Scientology. And if it continues to be done, known as well as it is now that you mustn’t, one can only consider it suppressive — not just ignorant — as who now doesn’t know you wreck a Release by running past the floating needle?”

HCOB 2 August 1965
Release Goofs
(Tech Vol. VII, p. 631)