PH paper can have interferences. Oxidizers like bleach can mess them up. It doesn't take much (ppm). This should be rare but can happen around, say, industrial sites.
PH meters, around pH 5.5-7.5, can get drifty. This usually happens with pure water and is cured by adding small amounts of KCl. Above pH 11.5, don't trust a ph meter unless it's specially made for that range ( alkaline metal error).
Recalibrating is another issue, when and if needed.