Lets see I can think of a bunch:
The M1 rifle had a defect as when it fired the last round the "ping" of the ejecting clip would cause 30 or 40 Germans/Japs to rush the chap with an empty rifle. as if you could hear that or anyone would not worry about the other 7 or 8 man in the squad whose rifles were not empty.
The MG42 fired too fast...not if you understood Germandoctrine it didn't, and the Germans were the ones that used it....pretty darned effectively, it is the only WWII design still seeing active service nearly 80 years later (with a caliber change)
6.5 Jap MGs used a weaker cartridge than the Type 38. False, only took 45 years for that to be shown to be untrue.
The AR cannot be a winning 600 yard service rifle. Got disproved in 1994.
You cannot win the Finals at Bisley with anything other than a rear locking Enfield, disproved in 1986 by Tucker and a rather ugly Swing action. Of course it did depend on RG getting an improvement on the priming consistency.
The Jap 6.5 type 38 rifles were weak: often said until the famous 6.5/30-06 wildcat 1959 squeeze-bore showed otherwise.
Low number M1903 rifles were time bombs waiting to explode at any minute with safe M2 ball ammo. Strangely that was not the case in the USMC, the US Army in WWII and indeed they supplied reworked rifles to Greece in 1948.
Firing a few rounds of non corrosive ammo after a range session of corrosive ammo will blow out the fouling.
The G36 is way better than the M16series of weapons, all the experts said so in 1996~97. 23 years later the G36 is fading away and the M16 series seems to still be plodding along towards 60 years of active service.
The 300 Win mag is the unbeatable cartridge for the Wimbledon cup (any rifle/any sight)... was true for many years last century...Until it wasn't any more, now almost no one shoots one in serious 1000 yard competition.
The M1903 was the most accurate service rifle ever fielded...until the K31came along.
The K98K was really a hunting rifle in a military disguise…..No, it was a service rifle optimized primarily for snap shooting. Pretty effective in its intended role.
I am sure there are many more, those just popped into my head.