In Australia, it takes 30 poor competition rounds to increase my handicap by three shots. So, no good golf in say 7 months and I can increase from a 6.0 to a 9.0 (at 0.1 per bad round). A bad round is where I play at least two shots worse (compared to my handicap) than the 15th percentile (or so) scorer in the competition, which will be close to the course rating (unless conditions are really tough).
If 'I' put in less than 6 competitive rounds in a year (or so), 'my' handicap lapses. Five cards (or so) later and 'I' have a new handicap. It might be 16 based on these cards. People who play with 'me' might consider me a sandbagger, but 'I' have no choice. 'I' have followed the system. Of course, the Club (and Match Committee) could have been more diligent in issuing 'me' a new handicap. (note - my personal handicap has never lapsed, this is just an example).
From my experience, it is difficult for a long-term handicap holder to sandbag more than say three shots in Australia. However, a holder of a newish handicap can have the wrong handicap for a variety of reasons.
When I was 12 (many, many, many years ago), at the end of summer holidays I had a net 57, 15 under par for a Medal round. 82 off the stick. I was hitting a 2 wood to 195 yard holes, a 4 wood to uphill 155 yard holes etc. I won my grade by one shot, to a poor soul who shot the second best net score that I have ever seen at my club. I expect he was delighted with how he played, and was not decimated by missiing out on the $10 trophy voucher for the Grade winner. Neither of us were sandbaggers.
Now, if 'I' am playing in a big money game (or the King Putter, or playing some notable identity on GCA) then perhaps 'I' will seek extra handicap for that event. Perhaps 'I' wont.
James B