Thanks Adam! For your quick response and you were right on the money... running mysqlcheck solved my problem:
>mysqlcheck -u 140dev -p --auto-repair 140dev
140dev.json_cache
warning : 9 clients are using or haven't closed the table properly
error : Found 407095 keys of 407096
error : Corrupt
140dev.tweet_mentions
warning : 8 clients are using or haven't closed the table properly
status : OK
140dev.tweet_tags
warning : 8 clients are using or haven't closed the table properly
status : OK
140dev.tweet_urls
warning : 8 clients are using or haven't closed the table properly
status : OK
140dev.tweets
warning : 8 clients are using or haven't closed the table properly
status : OK
140dev.users
warning : 8 clients are using or haven't closed the table properly
status : OK
Repairing tables
140dev.json_cache
info : Wrong bytesec: 0- 0- 0 at
2052533412; Skipped
warning : Number of rows changed from 407096 to 407095
status : OK
... and we're back!