Poor choice of mounting holes: should be 3 mm, not 2.9 mm !
I have tested this myself: a properly drilled hole using a good quality 3 mm drill bit will accept a 3 mm machine screw with no problems whatsoever.
But a 2.9 mm hole will need 2.5 mm screws; these are hard to find and expensive, especially when the only place that sells them will add a substantial amount of shipping/postage and handling charges, and you do not wish to order anything else from them.
The Raspberry Pi foundation advices against re-drilling those holes with a bigger drill bit, and warns that doing so will damage the raspberry pi circuit board.
Question: Does this warning apply even to re-drilling with a 3 mm drill bit ? After all, the difference is only 0.1 mm, but would allow to use mounting screws that are easily and cheaply available, even in small quantities.
Like I have already written, 2.5 mm screws must be purchased in a plastic bag containing one hundred screws and the same goes for 2.5 mm nuts as well. And, since local stores do not sell this size, that will add postage fees as well, totaling over 11 euros just for two screws and two nuts for them !
Even big electronics stores like mouser.com, they do sell 2.5 mm machine screws in single quantities but NO matching 2.5 mm nuts: so just screws without matching nuts are useless! And, theit price for 2.5 mm screws is much more expensive than a local shop's price for 3 mm screws. The benefit here would be that if you order other products from mouser anyway, there will be no extra shipping costs for the screws; instead, if your order is at least 65 euros, they offer free shipping, and no handling fees anyway.
Just for information, here's the only shop's prices in Finland that sells 2.5 mm screws and nuts:
2.5 mm nuts, a bag of 100 pieces, 1.51 euros.
2.5 mm screws (threading length 12 mm), a bag of 100 pieces, 2.93 euros.
6.90 euros shipping and handling fees at minimum. (express shipping more expensive)
So, this means that just to get 2.5 mm screws and bnuts, a total order will be:
1.51 + 2.93 + 6.90 = 11,34 euros which buys you 100 screws and 100 nuts.
Prices above include the finnish VAT of 24%.